Prop/WOT/engine height - would appreciate your thoughts.

[Sunbeam]

I hope you can all put up with one more thread on this topic. But although I get the general concept (and have read through the archives), I'm still not sure what to change (prop-wise) on my individual boat, and I would appreciate any suggestions as to where to go, prop-wise.

So, first what I have now, and then some background to fill in more details:

Yesterday I went out for the first time in my "refit" boat. This being a 22 Cruiser, with Yamaha F80 main and Honda 8D kicker. The main has a Permatrim, and I have Bennett M-120 trim tabs. We were fairly heavily loaded, with about 15 gallons of water and 40 gallons of fuel, plus enough food for a couple of weeks or more, most of the tools I would ever have aboard, other usual gear, and two people (combined weight of about 400#). We were in fresh water at around 400' above sea level. Conditions were calm and smooth. Prop is a new Solas aluminum 4-bladed, 13 pitch x 13.25" diameter. (skip down a couple of paragraphs for comparative info). The bottom is gelcoat, although I waxed it recently (which I realize will slow it down slightly, but I did it for easier scrubbing of growth/slime).

Running at WOT, I was able to make around 5400 RPM and 20mph (currently have things set on statute miles as that is how it was for initial trial at purchase). It didn't seem to change a lot with various adjustments, but this was with engine trimmed fairly neutrally and maybe 30% tabs or so. I tried for getting the "spray trail" as far aft as possible looking out the side window.

My engine manual recommends a range of 5,000 - 6,000 RPM at WOT, so I'm within that range, but I would rather be closer to the upper end. Also in my mind is that I'm about to head to Powell, and so things can only get "worse" there. I wonder if I should get another prop - could be a spare/Powell prop.

Some background:

When I sea-trialed the boat briefly at purchase, things were a bit different. The boat was very light (no fuel, 15 gallons water, no gear, two people combined around 280#). At that point I had no Permatrim, no tabs, and the prop that came on the boat, which was a 3-bladed aluminum Michigan Wheel, 15 pitch x 13.5" diameter. We made 5600 RPM and 28 mph at that time. This was on a freshwater lake at about sea level, a very light chop and wind.

So, in a nutshell:

Then
Light boat
Sea level
3-bladed 15 x 13.5" aluminum prop
No Permatrim or tabs
5600 RPM/28 mph

Now
Heavyish boat
400' altitude
4-bladed 13 x 13.25" aluminum prop
Permatrim and tabs
5400 RPM/20 mph

I consulted with someone more experienced than me to choose the prop, and I'm guessing that with the original prop things would have been much worse, but I still think I'd like it better for sea level running, which would make it "less worse" on Powell too (which I'm about to leave for). I'm fine with getting another prop, but just not sure what to get, either in number of blades or other dimensions. Can someone make a suggestion?

*****************

The other factor I wonder about is engine height. Roy & Dixie from C-Way have a 22 that's three years older than mine, but with the same engine and from the same dealer. They found that their engine was set too low on the transom, and moved it up around 1-1/2" as a result. I'm not positive, but I think mine is about 1" higher than theirs was. Ultimately it just matters how it is on my boat, but their findings did raise the question in my mind about that dealer maybe setting engines too low.

From what I have read in the archives, a first, "loose" check is to see if the bottom of the anti-ventilation plate is level with the bottom of the boat. Mine is an inch or so lower than that, I think (I wasn't sure exactly how to angle the engine vis-a-vis the bottom or the transom; I guess the gap would be smaller if I had raised the engine to make them more parallel). You can see it here:



I also read that a more precise way is to see that the Permatrim rides just at or under the surface of the water when up on plane. I tried to ascertain that yesterday, but there is a lot of turbulence back there when on plane (is this normal?) so it was hard to tell. Maybe it's easy to tell when it is at the proper height and can be seen? Or does one always sort of "extrapolate" through the spray?



If the engine is too low, I imagine that is the thing to correct first. On the other hand, I'm not sure I would do that before Powell, and so I still think maybe I would rather order a prop now that might help. If I later move the engine up, it seems it would still be useful to have a "gradient" of props for different loads/altitudes, and they can be spares as well. If I can get one ordered tomorrow, I should be able to receive it and put it on before Powell. I'm hoping to spend a few weeks there and don't like the thought of lugging my engine that whole time.

Thanks,
Sunbeam

[thataway]

I thought that I was the only nut who took a boat on Crescent Lake! Hope you had lunch at the Lodge. We go up there every year when we are in Sequim!. I have taken both the 22 and the 25 up to Crescent, but we always had more chop than you had! Thanks for the great photos!

I have always had 90 hp outboards on the 22, but the 15" pitch is been what I used for sea level and 13" pitch for Powell. I think you have the Powell prop on the boat now, even if it is a 4 blade.

First, remember that these boats are very weight sensitive. But when light and well balanced should get close to 30--and that is what your boat did with the 3 blade when you were at sea trial.

Lets look at things. If the engine is properly trimmed, the Permatrim should be just at the surface of the water--yours is well buried. So your motor needs to come up a little (was it the 300 lb person or the 100 lb person taking the photo?--that could make some difference.

I would have fiddled with the engine trim and the tab trim to find my fasted WOT--Generally with the permatrim on the engine, I have the tabs pretty well retracted. the 30% tabs sort of puzzles me. You look at boat speed, not where the wake is breaking. For WOT trim the tabs all of the way up, trim the engine about neutral--and then with the throttle full forward move the engine trim up and down slowly to see where you get the maximum speed of the boat--and that should also be the MAX RPM

Looking at the photo of measuring the level of the outboard--Have the cockpit floor or cabin floor level on the trailer, and then put a level on the Permatrim , and put the yard stick or what you are using on the bottom of the boat to establish a plane. The stick held against the bottom of the boat should show the Permatrim or top of cavitation/antiventillation plate should be at the same level as the bottom of the hull. Yours is not visible at all, showing that the motor is set a bit too low. I would take it up one hole and check again.

Who ever suggested the prop knew that you were going to load the boat--and suggested a lower pitch with 4 blades to give stern lift. Going from 3 blade to 4 blade may cost you from 100 to 300 RPM (depending of cupping etc.) So the 4 blade prop may be what you are going to get...but I am wondering about trim etc. and weight. Crescent Lake is about 500 feet so that should not make any different. My concern is that you have a lot of prop slip by calculation: 30% slip. The light boat was 18 slip--and that is a reasonable number. If your boat had gotten 30 mph light, the slip would have been about 12.5%, which is ideal. So maybe the motor being down a bit low, plus the drag of the permatrim in the water slowed the boat down.
(not saying the permatrim is bad, but it needs to be on the top of the water)

So you have some things to do--put the 300 lb gorilla on a diet, :-)
See what tools you can off load. Raise the motor one hole. I don't think that you are going to hurt the motor at Lake Powell, but you are going to be slower--if you are going to stick with 4 blade then go to an 11" pitch, If staying with 3 blade, then I would stay with 13" pitch. I might also try the boat with the 15" pitch prop that came with the boat.

Again--all of this shows the sensitivity of the boats to weight...

[Sunbeam]

A bit more info, and maybe I could get some comparison thoughts from you-all.

I called the place from which I ordered my current prop, gave him the scoop, and asked for his thoughts. In somewhat random order:

- I did go the right direction in prop selection; with my original prop I would have made a lower RPM now.

- 5400 RPM isn't too bad, but that's a big drop in speed.

So, fellow Brats: Is that kind of a drop in speed something to be expected going from a lightly loaded boat to a heavily loaded one? (28 mph to 20 mph).

- There may not be a logical "slight improvement" size of prop, but he could potentially tweak my current prop; however that would give me more RPM at the expense of top speed (not that top speed is a priority for me at all)

- 4-bladed is usually good for a "heavier" boat (I asked if I should have stuck with a 3-bladed one).

- As far as engine height/looking at Permatrim it is also a "feel" sort of thing. (I certainly don't have that expertise yet.)

- I would lose RPM at altitude (Powell) with this prop, but there is no "easy" next prop to move to. It would need to be an 11 pitch; he is going to look around for options and call me back. This is the prop I have now:



My thought now (which may change as I learn more) is that since my "sea level" prop is not so bad that I'm really lugging the engine, I will wait to do anything to it until I get a bit more experience running the boat, and perhaps figure out engine height. But I will likely order a different prop for Powell now as this one is obviously not going to be optimum. I don't yet have a target prop to order though.

[thataway]

There are few 4 blade props for that outboard, because few folks use the 4 blade prop--most use the 3 blade. Ken at Prop Gods is the Go to guy about props.
http://www.propgods.com/Content2/Default.aspx
I consider him to be the best in the country. If you were near his shop he would give you several props to try. He told me to go to 11" pitch on the c Dory 25 for powell--and it did the job. Picked up 5 knots on top speed. Barely on a plane to full on good plane.

The reality is that most of the C Dorys are running 3 blade props. We get in the low 20's cruise speed with a loaded 22 with a 15" stock prop and Honda 90. You noted our speed--about the same as Les's boat at Powell, with a 3 blade 13". C Cory's will run fine any where from 14 to 20's Look at fuel efficiency.

[Sunbeam]

I see I replied earlier but it came in after your post - which I had not read at that time. So maybe it looked like I was ignoring your (valid) request for info. Let me respond that now.

[Sunbeam]

So here is one reason I'm confused. I searched "Powell" and "prop" and was happy to see a post by Kerri-On. Reason I say that is that there are two people living on the boat, so it must not be super light, and he has a Yamaha 90, which at least bears some resemblance to my 80 (although mine is really the equivalent of the 75 in current parlance). Here is what he had to say (and George, if you are reading, I'd be very interested to hear your thoughts):

[thataway]

I am trying to figure out why your boat is behaving the way it is. I carry a Waeco freezer, extra group 31 battery, , Full ice chest, 5 gallons extra water, 3.5 hp outboard, plastic box (contains propane stove, grill, Honda Generator, extra anchor) in the cockpit and inflatable on the top, plus clothing for a month and food for a month, Marie, myself and the dog are about 330 lbs. and seem to be running better than you are (and I also compare the first 22 I owned which had no Permatrim or tabs, as well as the current boat with a Doel Fin and Lenco tabs). In that I don't seem to have a problem with bow up so high, I don't have problem with side spray....How did the boat run on sea trials ref the spray and bow attitude? Maybe you are running with the bow down too much? But you say that you have tried different combinations....

The reason I suggested to go back with the first prop to compare the amount of slip. The slip is way high currently @ 30% [assuming that I have the correct lower unit ratio (2.31)] With the 4 blade 13 pitch prop.

With the 25 and Tom Cat, you could see the Permatrim on top of the water. The current boat has a Doelfin--which, if I get well, I will change out for a Permatrim.

I probably would go with the prop you have to Powell and move the motor up. Try one hole at a time. Run, than go back--unless it is obvious when you measure that you need to go up two holes. If you want to buy another prop, either a 3 blade 13"pitch or a 4 blade 11" pitch. I cannot find a 4 blade in 11" for your engine--but it may exist. But I would give Ken at Prop Gods an e-mail.

This is from Boat Test. com:

Take two propellers, identical in design (blade shape, diameter, rake, cup, etc.) that is appropriate for a given application, and simply add a propeller blade, we get a truer representation of just where the difference lies. The addition of the extra blade causes increased drag, which, in turn, requires more horsepower, in order to achieve the same rpm. Since the horsepower is limited, the rpms drop, and the speed will tend to drop with it. This is why, when going from a 3-blade to a 4-blade, the pitch is dropped an inch, or more, in order to keep rpm parity. It is this difference in pitch that causes any potential speed differentials between the 3-blade and the 4.

[digger]

One other thing of note...when you have a slight chop, the boat is faster...takes some of the drag off the bottom. This is especially noticeable when you are at a near critical point of the power range. On my boat, I found the fuel consumption and speed were both improved by having the bow of the boat slightly downward. It made the engine feel like it was about to start laboring. Also, it would help to start the test by not having the tabs deployed -- just trim with the motor at around 4800 rpm, and find the trim that gives you the most speed. Then slightly bring the tabs down until you determine if the boat speed is improving or slowing. Find the best speed with that adjustment, and tweek the motor once again and see if you can improve the speed by adjusting the motor trim up slightly. Make all the adjustments without changing the throttle setting.

[Sunbeam]

[KickerDRB]

Glad you got on the water and enjoyed the fruits of your summers labor. Sounds like you are getting to the bottom of your prop dillema.
As far as motor height, I thought the best time to tune or adjust was when the boat was near the lightest state. It would be a real bummer if you tuned the system for a heavy load, then ran it one day by yourself (without full up water and fuel) and the prop was cavitating away.....

regards,
Dave

[Sunbeam]

[Sunbeam]

Okay, I'm posting back to myself now , but I found this link in another thread here (while searching archives) and just now remembered that I had it open in another tab. Okay, so the "buried/too low" AV plate photo sure seems to look a lot like mine.

Their photo of "too low" AV plate:



My engine (I think we were at 4300 RPM and 16 mph here; they say to "take it up to cruise," so I think this should qualify:



Kinda looks like the "too low" one to me - do you think?

And the "proper" one? I never saw anything like that - it would have been pretty obvious with that big Permatrim up that high! I would have assumed that was too high before having read/learned more (here and at the link).



I'm a bit uncertain about the recommended loading for testing. They say to test engine height at 50% fuel, and I had one full tank and one just over half full at that point, so 75% fuel. On the other hand, I didn't have a cooler/ice/food in the cockpit, and I wouldn't be surprised if that weighed 100# (or about 16 gallons of fuel worth). Also I didn't have the kicker tank along (3 gallons, for another 20#).

But then for testing WOT RPM, they say 50% fuel and "a light load for fishing. cruising, etc." So I'm not sure if they meant to say that for the height test and just left the second part off, or what. So I don't know if the engine height should be tested at fullish load or at 50% load...

If they really meant 50% fuel, but normal load (which for me would be heavier as I figure I'll mostly cruise) I was probably at that equivalent. If they meant 50% load overall (because not talking about boats typically loaded for cruising), then I'd have to take stuff off the boat, as I was probably closer to 80% overall load for cruising.

Anyway, seems like a really good link (and also refers to the Prop God).

http://www.veradoclub.com/smf/index.php?topic=137.0

Sunbeam

[tpbrady]

On both C-Dory's I have owned, the engines were mounted too low by the dealer by 1 1/2 to 2 inches. In both cases, I wish I had more upward adjustment but ran out of transom. I used Permatrims on both. On the 22 the Permatrim didn't seem to make a whole lot of difference other than cutting the top end speed by about 2 mph. Yes it helped get the bow down (or stern up), but its main impact was allowing a pretty efficient cruise speed at 12-14 mph. With the Permatrim off that cruise point move up a couple mph which is a lot rougher in a 22. On my 25 the Permatrim was a real asset as that boat is just plain stern heavy and it made a lot of difference in trim. In both cases I could never get the motor high enough to get the Permatrim up to the surface as far as it should be. In your case, with the Permatrim buried, it represents more drag than anything else. Moving it up will reduce the drag both from the lower unit of the motor and the Permatrim.

Tom

[Sunbeam]

[Chris]

I run a 13 1/2 X 15 three blade all the time. It is the sweet spot where I get good performance at all elevations and loadings. I loose about 200 rpm at Powell but it really doesn't hurt because I'm still turning 5400-5600. It's important to let these motors turn up. I think the goal should be to prop so that you are turning the high side of your rpm range at sea level while light. Then when you go to altitude or load you will loose a little rpm but still be in an acceptable range...until you go extreme load or altitude. The dealer propped my boat with a 17 which I knew would not work once I loaded the boat. Dealers do that because they are dealing with boats that are new and light.
Your motor mount looks OK to me...looks real close to where mine is. Being a little low is definetly preferable to too high.
You absolutely should have a spare prop at Powell. File and a hammer a good idea also. Those rocks can be sneaky!