I'm not a stupid guy but, prop numbers have me stymied. Talk to the shop guys and it all makes sense right up untill I hang up the phone! 16 Cruiser with EFI 50 Honda, current prop is stock 11.1x13 three blade aluminum. Under our usual load the motor will come up against the rev limiter before WOT. I would like to gain more push/faster acceleration and lose a few top end RPM's. Top end speed not an issue, we do not need to go so fast! Can anyone tell me what numbers I should be shopping for to achieve this goal? Many thanks in advance.
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My Apologies: Props Again
- Thread starter ramos
- Start date
Jon,
Wow your "problem" is a nice one to have!!!!
First of all the 11.1 is the prop's diameter in inches. The 13 is the pitch or how far in a perfect world you would travel in one revolution of the prop. In water there is a lot of "slippage", so that distance is generally less.
If you are "coming up against the rev limiter before WOT", most would say that your prop could have more pitch. You should already be experiencing excellent "hole shot" and your top end is already being reduced from what it could be.
I suppose you could drop to an even lower pitched prop (from a 13 down to an 11 or 12) but will encounter the over rev situation even sooner on the throttle. If that is what you do, be very careful. If your rev limiter fails you could easily burn up your engine.
You might want to try a larger diameter prop at 13 pitch and see how it behaves. You will get some additional "bite".
Hope this helps.
Capt Dan
Wow your "problem" is a nice one to have!!!!
First of all the 11.1 is the prop's diameter in inches. The 13 is the pitch or how far in a perfect world you would travel in one revolution of the prop. In water there is a lot of "slippage", so that distance is generally less.
If you are "coming up against the rev limiter before WOT", most would say that your prop could have more pitch. You should already be experiencing excellent "hole shot" and your top end is already being reduced from what it could be.
I suppose you could drop to an even lower pitched prop (from a 13 down to an 11 or 12) but will encounter the over rev situation even sooner on the throttle. If that is what you do, be very careful. If your rev limiter fails you could easily burn up your engine.
You might want to try a larger diameter prop at 13 pitch and see how it behaves. You will get some additional "bite".
Hope this helps.
Capt Dan
Capt Dan,
Thank you for the reply and yes, it does help me to better understand. I had not thought about changing diameter. I was assuming (theres that word) that if I increased the pitch, the motor would have a greater load on it thus reducing RPM at WOT. As an aside, we almost never exceed 5500 RPM by choice. Usual cruise for us is 4200-4800.
Thank you for the reply and yes, it does help me to better understand. I had not thought about changing diameter. I was assuming (theres that word) that if I increased the pitch, the motor would have a greater load on it thus reducing RPM at WOT. As an aside, we almost never exceed 5500 RPM by choice. Usual cruise for us is 4200-4800.
Matt Gurnsey
New member
To increase hole shot, we usually go down in pitch so the prop can "spin up" faster, but this usally crates a motor that could over rev, and loses some top end speed.
An increase in pitch increases top end speed (to a point) drops rpms at WOT, but can cause some reduction of hole shot.
Going to a four blade prop can increase mid range performance, but may cost some top end speed as the extra blade creates additional drag at upper end rpm's.
It is also possible to "ventilate" a prop to allow it to "spin up" faster, but still hgave top end speed and avoid the chance of over revving.
Prop choice is sometimes as much art as science, with a bit of alchemy thrown in.
Bottom line is that if you are constantly running up against the rev limiter, you may not be loading the engine properly at lower rpm's, reducing performance and possible increasing fuel use.
An increase in pitch increases top end speed (to a point) drops rpms at WOT, but can cause some reduction of hole shot.
Going to a four blade prop can increase mid range performance, but may cost some top end speed as the extra blade creates additional drag at upper end rpm's.
It is also possible to "ventilate" a prop to allow it to "spin up" faster, but still hgave top end speed and avoid the chance of over revving.
Prop choice is sometimes as much art as science, with a bit of alchemy thrown in.
Bottom line is that if you are constantly running up against the rev limiter, you may not be loading the engine properly at lower rpm's, reducing performance and possible increasing fuel use.
Jon-
I'd add 2 inches of pitch with pretty much the existing diameter or slightly larger, as a first alternative, and then see where you are.
Each inch of pitch drops the WOT about 200 rpm (general rule).
You don't give hard numbers, so that's the best recommendation under these circumstances.
Props usually come in 2 inch increments, but you can find one inch alternatives if you search around, or have an existing prop re-pitched.
Good Luck!
Joe. :teeth :thup
I'd add 2 inches of pitch with pretty much the existing diameter or slightly larger, as a first alternative, and then see where you are.
Each inch of pitch drops the WOT about 200 rpm (general rule).
You don't give hard numbers, so that's the best recommendation under these circumstances.
Props usually come in 2 inch increments, but you can find one inch alternatives if you search around, or have an existing prop re-pitched.
Good Luck!
Joe. :teeth :thup
Joe nailed it ! Watch out for adjustable pitch/plastic/composite stuff . They are usually horrible performers . 2" up in pitch and close to the same or same diameter and you should be right on . Keep the one you have now as a spare or for towing the grand kids on tubes .
Marc
Marc
Joe & Marc,
Okay, I've been a lurker and member long enough to know when experienced knowledge has been spoken (typed). I will take these sugestions to heart and purchase accordingly! If you have anything else to add, please feel free. I will not be able to check in until Sunday as I have a cousins wedding to attend to out of town. Towing grandchildren will have to wait (I hope) as my two boys are 5 & 6. They will be the first two towed behind C-squirt. They think they are ready but I'am waiting until next summer!
Okay, I've been a lurker and member long enough to know when experienced knowledge has been spoken (typed). I will take these sugestions to heart and purchase accordingly! If you have anything else to add, please feel free. I will not be able to check in until Sunday as I have a cousins wedding to attend to out of town. Towing grandchildren will have to wait (I hope) as my two boys are 5 & 6. They will be the first two towed behind C-squirt. They think they are ready but I'am waiting until next summer!
Although most of the props for your motor are 10 1/2 to 11.1 inch diameter, there is one SS prop listed at 12" diameter (3 Blade Stilletto).
Part of your problem may be that you are using an aluminum prop. Sure they are much cheaper, but they do flex more. (The composites even more).
What you want to look at, is to take the boat speed up slowly, and record RPM vs speed (GPS). See if there is a lot of slip--and see if there is "ventillation" (somewhat similar to cavitation)-- and all of a sudden the RPM goes way up. That means that the prop is loosing the "bite" on the water. This may be resolved by repositioning the motor height, changing water flow (moving depth sounder transducer etc). You also have to look at the amount of cup in the prop (let alone rake and cleaver--which probably are not real important in your boat/engine set up). But these additional factors can made a real life difference in performance.
Look for that slipping. I am a bit surprised that you are hitting the rev limiter (which should be at 6,000 I believe) rapidly--and that suggests to me some ventillation...
Part of your problem may be that you are using an aluminum prop. Sure they are much cheaper, but they do flex more. (The composites even more).
What you want to look at, is to take the boat speed up slowly, and record RPM vs speed (GPS). See if there is a lot of slip--and see if there is "ventillation" (somewhat similar to cavitation)-- and all of a sudden the RPM goes way up. That means that the prop is loosing the "bite" on the water. This may be resolved by repositioning the motor height, changing water flow (moving depth sounder transducer etc). You also have to look at the amount of cup in the prop (let alone rake and cleaver--which probably are not real important in your boat/engine set up). But these additional factors can made a real life difference in performance.
Look for that slipping. I am a bit surprised that you are hitting the rev limiter (which should be at 6,000 I believe) rapidly--and that suggests to me some ventillation...
My 16 came equipped with a Honda 50 and an 11.6X11 original prop. I had major problem with cavitation on takeoff and high rpms 6000+. The dealer sold me an 11.1X13 and I kept the other as a spare. It stopped the cavitation on takeoff but, the rpms were still high. I then found out that the tach was on the wrong setting. It was on 3 and needed to be on 1. Now the high side of the rpms is around 5300-5400. My 16 is on the heavy side and with the help of trim tabs and a Permatrim, I can get 22 mph upwind/upriver and 26 mph downwind/downriver. If you search the web, you can find some prop calculators. Might not be as helpful as the old salt you can find on this site though.
ramos":1ov0h0zk said:Thank you all for your' input. A quick look on the internet gave me these options:
11.1X13 (original)
Alum. 11X15 3 Blade
S.S. 11X15 3 Blade
12X15 3 Blade
Alum. 10.25X14 4 Blade
11X14 3 Blade
Anybody want to pick?
ramos-
I'd be sure to do what Bob says to check out the ventilation slippage first, then if you find nothing unusual, my 1st choice would be the S.S. 11x15, which the aluminum version of those dimensions my 2nd choice, from what you've told us. But remember, no one has a magic crystal ball in world of props! And 15" of pitch does see a bit high for a 16, generally.
Can you find a dealer that will let you try out a couple of them on a free exchange basis as long as you eventually buy one or the other?
Joe. :teeth :thup
I know there is not a definate way to buy the perfect prop without actually mounting it and running it. I just appreciate all of the feed back. Each post has given me a little more information and knowledge that I did not have before. I believe my odds of picking an acceptable prop is higher than it was before this post. Thanks to each of you.
Google it Find a prop repair place ,marina etc .Go and try a few props usually a good prop place will let you try as long as you dont bugger them up . Go with a S/S prop better hole shot , top speed and economy and try not to hit anything . put on the boat what you usually take on fuel gear etc then run engine up to redline rpm should be about 5800-6000rpm .If your eng cavitation plate is not even with bottom of boat you might have to reposition engine Good luck
I think Honda changed the gear ratio on the new design outboards , which may explain being able to run a 15" pitch prop. The old motor[carb] couldn't do it . They also have a very punchy mid range with the "lean Burn" .
The Stilettos are highly cupped which can lead to porpoising . 16 C Dorys run much better with a Permatrim from my experience and will help control cavitation and porpoising.
Marc
The Stilettos are highly cupped which can lead to porpoising . 16 C Dorys run much better with a Permatrim from my experience and will help control cavitation and porpoising.
Marc