OK I am still confused on the prop/engine

R

rainger

New member
So I have read that the boat should be powered along at between 25 and 30 MPH. I get it. I also get conditions and engine configurations make a difference. Plus I have tried to read every thread in this topic.

Here is the thing I am unsure of. When I sea trialed my new boat the guy put the engines all the way down and did not trim them up but that I don't think is the issue. At WOT the boat was only showing 5000 and a top speed of 17.5K. Now I have twin 40 Hondas that are rated between 5000 and 6000.

When he sold the boat he had some extra props he had taken off cause oh "all they did was spin" ???? The props he took of were 10.5 D by 13 P
He replaced them with 11.5 D 10P.

So 17.5K at WOT (5000) is only about 21MPH. I am confused. One guy tells me I need to move to even less pitch to bring up the RPMs, and another guy tells me I need to increase the pitch to push more water for a higher speed.

I would have thought that with twin 40s at say 5500 RPM I should get like 25 to 28MPH.

Is it the prop? Is it the trim? Or as one guy told me "oh 2 40s are really only 60HP not 80HP and 20MPH is all you can expect".

Any sage wisdom would be helpful.
 
I can't give sage wisdom (others here probably can), but I can say that you absolutely want to get the rated RPM at wide open throttle (WOT). If the range is 5,000 to 6,000, then at sea level it's good to shoot for something above 5,600 if possible. The reason for this is that otherwise you'll be lugging the engine (even at slower speeds). Others can explain this better, but essentially you don't have a transmission, so making sure the engine makes it's rated RPM at WOT is the way you keep it from lugging (even if you "never go fast").

A few things I would check:

1) Is the bottom clean? By that I mean no excessive paint, no barnacles, little slime?

2) Did you try an "out and back" to eliminate possible wind or current factors?

Next, while underway I would get to a given speed and RPM (on plane at a higher speed) and then play with the engine trim. When I'd get to the highest speed (without changing the throttle) then I'd presume I was trimmed advantageously and do the WOT RPM check again.

3) Others can advise you on which way to go with your props, but first (if it were me) I'd probably do the above steps just to re-check for yourself (vs. the previous owner on the sea trial).

4) Once you get the right props on (so you get the recommended RPM at WOT), then your max speed will sort of be what it will be, and will depend on loading, etc. But I would expect something between probably 22-30 knots. Some boats don't make the lower speed (even) when they are heavily loaded, but I don't imagine you would be loaded like that now or on sea trial.

I have a single Yamaha 80, and on sea trial - with a light boat and at sea level - we made 5600 RPM and around 28 knots (my engine also has a 5,000 - 6,000 WOT RPM range). I have since been playing with props/trim as I now carry more load, have a Permatrim and tabs, etc. I also ran with a different prop at Lake Powell, because it's up near 4,000' and that affects things.

PS: This may be obvious, but with twins you run them together and prop them as "half of a pair." In other words, you run them both together as if they were one engine, and prop accordingly to get the WOT RPM. This is as opposed to if you were running a single 40 and so would prop it alone as a single.
 
PS: This may be obvious, but with twins you run them together and prop them as "half of a pair." In other words, you run them both together as if they were one engine, and prop accordingly to get the WOT RPM. This is as opposed to if you were running a single 40 and so would prop it alone as a single.

This would suggest to have say a 12D x 14P I should have each engine propped with 6D x 7P
 
rainger":a4au77ob said:
PS: This may be obvious, but with twins you run them together and prop them as "half of a pair." In other words, you run them both together as if they were one engine, and prop accordingly to get the WOT RPM. This is as opposed to if you were running a single 40 and so would prop it alone as a single.

This would suggest to have say a 12D x 14P I should have each engine propped with 6D x 7P
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I am not a knowledgeable propeller guy, but I suspect cutting the diameter in half but leaving the pitch alone would be a closer choice. If you decrease both by a factor of two, then the effect is 2 x 2 or a factor of 4. Likely this is not quite right either, because decreasing the diameter actually decreases the swept area of the prop by a factor of the diameter change squared.

There should be prop tables for this. Someone with more knowledge should know and comment.
 
We have run both 4 blade and 3 blade on our twin 40's. They are propped as if they were singles. Right now we have 4 blade props on. They let us run at our normal cruising speed (12-14NMPH)about 200 rpm lower than with the three blade. The tradeoff is in top speed but we never go that fast anyway. Our props are 11.1x11.
 
colobear":1pczlvrz said:
Are the props 3 or 4 blade? Are you reading speed in MPH or NMPH?
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4 blade and the GPS was in knots. WOT was only 17.5K

I am going out on Monday and will try different trim positions etc but I can imagine that is going to add 8 MPH more.
 
colobear":y6fq4log said:
We have run both 4 blade and 3 blade on our twin 40's. They are propped as if they were singles. Right now we have 4 blade props on. They let us run at our normal cruising speed (12-14NMPH)about 200 rpm lower than with the three blade. The tradeoff is in top speed but we never go that fast anyway. Our props are 11.1x11.
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OK then that would sound about the same as mine. I was getting 17.5 NMPH on WOT. If I was to back off 4500RPM I am sure I would be in that 12-14 range you mentioned. And my props are 11.5x10 and as if they were singles. Thanks
 
Don't worry C-Hawk, you are not the only one confused. Being an ex-sailor I'm not used to outboards, prop pitch, engine pitch, trim tabs etc. This may sound lame, but I'm not really interested in tweaking everything here and there in order to get another couple of mph. I find I go quite fast enough and my fuel consumption is acceptable.
I always seem to have my boat heavy, full fuel tanks and water, loaded up for cruising at all times. I do have anti-foul paint on the bottom, but it is fairly clean. I have twin 40 Yamis. WOT is 5000 RPM around 24 mph. I'm ok with that.
Maybe one day at a gathering I will ask someone to explain it all to me, if I'm sober :lol:

Martin.
 
Each boat is going to handle a bit differently, I have only owned 22's with single 90's. In these smaller boats, 10 hp can be significant.

There is the "Boat Test.com with a single 90 Honda--the same engine as I currently have:

cdory22-chart.jpg


The boats in Boat Test, tend to be light, with a perfectly clean bottom.

In this test at 3000 RPM speed was 9.4 mph 8.1 knots and 3.8 mpg
At 3500 RPM speed was14.2 MPH 12.3knots 3.9 mpg
At 4000 RPM speed was 19.1 mph, and 16.6 knots, 3.3 mpg
At 4500 RPM it was 24.7 mph and 21.5 knots 3.1 mpg
At 5000 RPM 27.4 mph and 23.8 knots. 2.9 mpg
At 5500 RPM 30.9 mph and 26.9 knots 3.0 mpg
At 6100 RPM 32.9mph and 28.6 knots 3.0 mpg

With much weight bottom paint etc, all of those numbers are going to change somewhat….probably not as low a planing speed and worse mpg.

This shows it is not about speed, but that mpg are fairly consistent.

I ran my boat in 6" chop and about 10 knots of wind on the beam a couple of days ago, and was running at 4200, about 17.5 knots. So that fits right in the specs which were on Boat Test.

Trimming all of the way down, is an issue--some folks run this way--but it will not give you top speed, nor best efficiency in my experience. For WOT testing, you want to trim the outboard for max speed and RPM. Usually this is with a bit of a bow up attitude. So until you fool around with various trim states, I would not make any changes. I suspect that your boat is propped correctly, and you are fine. How about trim tabs, were they down, up, or non existent/ My feeling is that the boats need trim tabs, and foils (Permatrims) for bet performance. My first boat had neither, and I was not all that happy with it. The second boat was a 25--and it was OK with trim tabs, but much better with both. Current boat has both. Rides better going into chop. Rides very well in heavy conditions down wind.

Generally if you drop the pitch 2" you should pick up 400 RPM. I believe that the 40's and 50's are the same blocks--the guy who told you 30 hp is smoking something…Same for the person who told you to increase the pitch. You are turning at the lower limit of WOT, but first trim the boat properly, and see what she does. Don't change anything until you do that.
 
We also have the 22 CD with twin Honda 40's. Most of our boat use is either very heavy for extended cruising at 0 elevation on the ocean or moderately heavy at high elevation such as Yellowstone Lake's almost 8000 ft. Through the 12 seasons of using this boat, we've been through a variety of prop types & sizes & have finally settled on the New Solas, three blade, 12dx10p stainless props from the last before of a pro pulse 4 blade. For us on the ocean depending on fuel & water load, weight change can be close to 800 lbs & the rpm will vary from 5300 to 6000 & wot speed from 18 to 22 in statue mph. This is from memory so nothing exact here. As Bob mentioned about the Permatrims & trim tabs improving efficiency, I believe it to be especially true at my prop size & resulting reduced wot speed. Permatrims & trim tabs have the most pronounced affect when loaded heavy with much of the extra weight aft by being able to best trim for speed with it taking considerable adjustment for me at times on what trim positions result in highest speed at a set throttle & then keep changing as weight & its position in the boat is varied.

I believe the 12 inch diameter of my present props, which are about the largest that will fit on the Honda 40, helps slightly with the small 10p for displacement speed running.

At a medium altitude for me like Lake Powell, where I run fairly light I find my present prop pitch to low & think the Solas three blade stainless in a 12dx11p would by the perfect match for me. Due to already having 4 other sets that are to high pitched for there, making the prop budget already strained, I'm going to make do with my present set up for now, but I think for others who don't cruise with 100 gal of fuel & very heavy loaded otherwise, the New Solas, 3 blade stainless, 12dx11p a very good prop choice for twin Honda 40's for Lake Powell & for ocean cruising.

We, similar to Barry & Patty, when on plane, prefer to cruise at approximately 11 to 14 kph & it seems his 4 blade & my 3 blade both work well for this.

Jay
 
colobear":2hqecaor said:
... our twin 40's... are propped as if they were singles.
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That's interesting. I always thought twins would be propped running together as if they were one (in other words, you'd have both engines running at WOT together and prop them to achieve the proper RPM when running together like that). In fact, I thought that was why "one twin" doesn't to the best job of planing on its own (such as if one breaks down and you are getting home on one); because it is propped as "half a twin," whereas if it were propped as a single it would plane the boat better (albeit slowly). At least I know a single 50 would plane the boat at around 15 mph or maybe knots (as per "Duck"). Of course it was propped as a single since it was a single on that boat. Anyway, that's not what I expected in terms of how one props twins on the 22 (that they are propped as singles). Sorry rainger if I steered you wrong by saying to prop them as a pair at WOT.

bridma":2hqecaor said:
This may sound lame, but I'm not really interested in tweaking everything here and there in order to get another couple of mph. I find I go quite fast enough and my fuel consumption is acceptable.... I have twin 40 Yamis. WOT is 5000 RPM around 24 mph. I'm ok with that.
Maybe one day at a gathering I will ask someone to explain it all to me, if I'm sober
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I realize you're not that interested, and I am not the best at explaining it, but.... getting into the right range of RPM at WOT is not about "tweaking in order to go faster." (Maybe you said that in response to the speed question though, and if so, then disregard this part of the comment.)

You know how in your car, if it's a manual transmission, if you, say, try to start in second gear it doesn't work very well because the engine is lugging? Or say if you put it in overdrive, drop down to 40 mph, and then punch it and try to pass without downshifting... also lugging. That's not good for the engine. Well, the outboards don't really have "gears, " so (and this is the part I'm not good at explaining) they sort of use the props to make sure they are always in the right "gear." If you can achieve the right RPM range at WOT (and preferably in the upper half of the range - maybe 5,600 to 6,000 for your engines (but double check that as I don't specifically know), then you are not lugging the engine at ANY speed. But if you can't get the engines up to the right RPM at WOT, then you are sort of always in the wrong "gear," and thus you are lugging the engine even at lower speeds (although you can't really tell). That's not good for the engine. So you check the setting at WOT, but it has nothing to do with running a high speeds or cruising at lower ones.

I've often heard people say something like "Well we can only get to 4,500 RPM at WOT (when they should be up around 5,600 or more), but that's fine as we don't cruise at higher speeds." So, unbeknownst to them, they are lugging their engines even at those lower speeds.

The way to get "into the right gear" (i.e. achieve the rated WOT RPM) is to tweak the prop.

I'm sure someone else could explain it better, but that's how I understand it at any rate (and it's not just outboards; inboards also get run at WOT to make sure they can get up to the rated RPM).

thataway":2hqecaor said:
Trimming all of the way down, is an issue--some folks run this way--but it will not give you top speed, nor best efficiency in my experience.
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It must depend somewhat on engine/boat set up. If I get up to a decent speed and on plane, and then note my speed and RPM, I find that if I trim the engine all the way down it is the quietest and I get the most speed per RPM. If I trim it up at all, it gets noisier and I lose speed. This is independent of the trim tabs, which do have varying effects at different settings.

I am wondering if this is because my engine is mounted a bit low (I do have a Permatrim). I have looked at a number of other 22's, and it's not mounted appreciably lower than they are (looks like the various riggers put the four holes in a similar place on all the 22's), but OTOH Roy, who has my same engine/position on his 22, raised it up by two holes and saw favorable results. I'm going to try that. Because it seems like trimming the engine up SHOULD have favorable results, but it doesn't. I have not found any condition in which there is anything better than having it trimmed all the way down.
 
When I say the engines are "Propped as if they were singles" I mean each 40hp engine is propped as a single 40 hp engine, not as half of an 80hp engine, after all, that little engine can only support a certain size prop. In other words, If I had just one 40hp engine I would have the same 11.1x11 4blade prop on it as I do now, since I have two of those engines I have two 11.1x11 4blade props. They are propped together for WOT purposes but when I do that both engines have virtually the same WOT rpms. Not sure if this is clear but its the best I can do.
 
Thanks for explaining further. I must have had it backwards all this time :oops: (Maybe lucky for my engine that I ended up with a single main!)

To the good, I guess that means that either of the twins would plane you home, albeit at a lower speed than two together (I had thought they would not do that due to being propped as half a twin vs. as a single.)
 
No...it means that the single 40 will still get to 5400 (or so) RPM but it will not plane my 22, that little engine and prop just doesn't have enough ooomph to do that. I have experimented and the one 40, at WOT will move the boat (heavily loaded) at about 10 to maybe 12 kts, not quite on plane. But, that is way too hard on the engine and I would not do it. The engine will move the boat comfortably at 8-9 kts at 4200-4500 rpm. I have tested that in a real world situation when the starter solenoid failed on one engine and we had to use the other one to go several miles to port.
 
colobear":2e8h9it1 said:
No...it means that the single 40 will still get to 5400 (or so) RPM but it will not plane my 22, that little engine and prop just doesn't have enough ooomph to do that.
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Okay, so then (still figuring this out in my head, so thanks for bearing with me) if one of your 40's running alone will make ~5,400 RPM (never mind the speed for now) --- what do they make for WOT RPM when both engines are running together?

(Actually, if the WOT for both together is 5,600 or more, then it would still seem like "propping as one of a pair" to me, which is what I was originally thinking. So I'm really curious now!)
 
about the same. WOT/prop is for that engine with that prop. It will still do that WOT with that prop regardless of how many other engines are there beside it.
 
Maybe this is off topic but I think when you get more familiar with your 22 you will be happy to cruise in the 20 to 14 mph range unless you are on water like glass. Think of the 22 like your Harley easy does it. :D
D.D.
 
Generally when a twin is run as a single, it will be "over propped"--that is lugging at WOT. The reason is, that when the boat is at WOT with twins, it will be on a plane.

If you propped the 40 hp engine correctly for a single, it would have a lower pitch prop, than when propped for twins (and planing).

Back to rainger's C-Hawk. Another issue is that the boat has bottom paint and enough marine growth to slow the boat down. Another reason to do nothing with the props, until the bottom is cleaned, old paint sanded smooth, or stripped, and a nice smooth coat of new paint applied. Painting may slow the boat a knot or two at the high end, but usually not enough to be an issue.

Also looking the photos of C-hawk, the props have some of the paint on the tips missing. I cannot see if there are any dings. I carry a hammer and coarse metal file, " just in case". It might pay to have these props scanned and trued--or you may not feel it is worth the trouble, if the blades are smooth, and no obvious sign of damage. Although it may not be as critical in the lower horsepower outboards, a scan will be sure the blades are properly pitched, and are identical, as well as balanced.
 
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