I must have wrong prop for F-25, grateful for suggestions.

[wet wonder]

I picked up a well cared for '75 F-25 Exress 3 years ago. Did a lot of work and everything is great - except I can only get it to 3k RPMs and 17 knots with the current and wind at my back. It's got a Mercruiser 5.7 233hp, which runs very well and stays really cool at 3k. I bought it thru a boat shop as the agent, but briefly spoke with the seller and recall he said something about changing the prop to save fuel or something. It was a passing comment and I only really just remember it. I don't know what I have on it now, but I know it can't be right. Any suggestions on what I should be looking for on a prop? I'll dive and swap it after I get this figured out.

There was an unused Dyna-Jet 13R10 standard taper hidden in the fore compartment when I took delivery of the boat. I don't know if that was the standard propeller. Would that be good?

Thanks,
"WW"

[prowlersfish]

The 13 x 10 should be real close is your eng a 5.7 chevy or a 5.8 ford ? I was thinking the 233hp was a ford but could be wrong . you should turning closer to 4000 rpm maybe 4400 if its a chevy at WOT .

make sure your tach is right


is the bottom clean ? if so you are way over proped

[WALTER GURAVSKAS]

I also hava a 1977 f25, I still have the factory prop. it is 14 X 10 rh.
The engine is a 318 chrysler 225 hp.

[wet wonder]

Yes it's a Ford built motor 233hp. I'd read about the 14/10 as maybe being the original prop, but I know I don't have the original spec'd engine. You think the 13/10 is right then? Or should I go 14/10?

Yeah, bottom's clean.

[ready123]

Yes it's a Ford built motor 233hp. I'd read about the 14/10 as maybe being the original prop, but I know I don't have the original spec'd engine. You think the 13/10 is right then? Or should I go 14/10? Yeah, bottom's clean.

If I were you I would use the 13x10 as you have it (if it looks good).... it will give you a little higher top end. If you run your boat heavy then the larger diameter prop would help get her on plane quicker.
If you are going to get a new one then go with 4 blades. I would test what you have first so if you are changing you can use that info to guide the new prop size and pitch. Check to see if the 13/10 gets your engine up to the spec max rpm. If it goes over then you can look at a bigger dia. prop.
From what other's with your boat have said here I think the 13/10 is it.

[prowlersfish]

The 13x10 is a good start . It will be closer then what ever you have how . If for some reson you go to a 14' and the RPMs where in spec with the 13 then you would have to go down in pitch to say a 14x9 to keep the same rpm .But if the rpm is hi then a 14X10 may be what you need . A 4 blade whould even need less pich . and to be honest a 4 blade wouls be a poor choice on your boat it would run better with a 3 blade IMO .


is the 13 cupped ?

Ready 123 we almost agreed on this LOL

[ready123]

A 4 blade whould even need less pich . and to be honest a 4 blade wouls be a poor choice on your boat it would run better with a 3 blade IMO.

Why do you think it will run worse with a 4 blade?
I consider that they get the boat on plane at a lower speed to be an advantage especially in heavy seas... the boat is more manouverable than a 3 blade. Docking is easier... yes you loose a little top end speed but who runs flat out for long anyway.
I'm interested in why you lean to 3 blade?

[prowlersfish]

A 4 blade whould even need less pich . and to be honest a 4 blade wouls be a poor choice on your boat it would run better with a 3 blade IMO.

Why do you think it will run worse with a 4 blade?
I consider that they get the boat on plane at a lower speed to be an advantage especially in heavy seas... the boat is more manouverable than a 3 blade. Docking is easier... yes you loose a little top end speed but who runs flat out for long anyway.
I'm interested in why you lean to 3 blade?

three blades seem to work better when you have high shaft speeds as you do when you have a 1 to 1 gear also a prop will change the planeing speed any big anount . to use a 4 blade on his boat and be able turn it you would need a 14x9 or a 14x8 pretty flat pitch .

a boat just like mine same engines (well 15 more hp each)
same gear is runs 2 knorts slower cruse and top end then me only difrence is he has 4 blades . you would think 4 blades would do better on a boat like mine but its not always the case .

as far as more beening manouverable I will have to take tour word on it now a 4 blade will be a lot smoother for sure .

the most important think is be in the right rpm range. The boat is over propped now not good for the engine at all .

The owner before change props to save fuel ??? over proping will not save fuel it may turn less rpm but it will burn more fuel at a given rpm if over propped .

Ever read Davied Gerrs bookm on props ? a lot of good info (to much LOL)

[kevin babineau]

prop sizeing can be frustrating to say the least....and costly...4 blade..3 blade..to cup or not to cup..lol...my opinion (which is only that)...u should run the prop whatever the guy posted the reply that he had the same boat as urs...unless u had the motor 'hopped up' and its putting out an extra 100hp or something....as for engine rpm specs...also someone here should let u know...people do change prop sizes 'thinking' that it will change speed, fuel economy, planing speed...sometimes it does but most the times it in their heads....with out knowing the specs it should whind up between 4200 and 4500 rpm but that is a guess..that rpm figure is important..engines have a power band...that means that there is a sweet spot where it preforms the best..by using the wide open throttle (WOT) as a reference puts the motor in that power band if u change the WOT figure ull be out of that preformance range creating operation under or over that sweet spot..

[ready123]

I'm not sure I agree with your argument... though maybe you need to have a coffee and tell me again, I've just had one too so maybe I will grasp your points better the next time, this response of yours has me a little confused.

three blades seem to work better when you have high shaft speeds as you do when you have a 1 to 1 gear also a prop will change the planeing speed any big anount . to use a 4 blade on his boat and be able turn it you would need a 14x9 or a 14x8 pretty flat pitch .

The flatter pitch makes the 4 blade easier to turn. Also allows it to be lighter and thinner blade section as the load per blade is less. The loss of pitch may well not be fully felt as some is gained back by less slippage of the 4 vs 3 design.

a boat just like mine same engines (well 15 more hp each) same gear is runs 2 knorts slower cruse and top end then me only difrence is he has 4 blades . you would think 4 blades would do better on a boat like mine but its not always the case .

as far as more beening manouverable I will have to take tour word on it now a 4 blade will be a lot smoother for sure .
the most important think is be in the right rpm range. The boat is over propped now not good for the engine at all .
The owner before change props to save fuel ??? over proping will not save fuel it may turn less rpm but it will burn more fuel at a given rpm if over propped .
Ever read Davied Gerrs bookm on props ? a lot of good info (to much LOL)

I agree he is over propped now why I think he should use the one he found in the bow locker. If that runs well he may have to do nothing... all depends what rpm he gets. I agree that with props one needs to make sure they give recommended engine revs @ WOT boat load normal.
I think our jury is still out on 3 vs 4 for the new prop though... let's continue this discussion.
I still maintain he will get on plane faster have a more stable boat and better economy with correct 4 blade but loose some top end speed and cruise at a lower speed. In these kinds of cruising boats I'm not sure top end speed loss is a worry.
My 3 to 4 blade switch dropped my cruise from 20/21 mph to 19 mph at same rpm but my fuel consumption went down. Also in heavy seas I can stay on plane at a lower speed which gives a more comfortable ride. My Admiral would have me slow down when I had the 3 blades and the off plane steering effort was a pain at the helm.
Just how I see it from my end.

[kevin babineau]

as for the 3 or 4 blade...my experience is that 4 blade has more torque and less top end.. how much more torque and how less top end...i dont know...but i do know that it is hard to find a store that will let u try them and if u dont want it u are able to bring them back

[prowlersfish]

"I still maintain he will get on plane faster have a more stable boat and better economy with correct 4 blade "

I say he will get on plane faster and burn less fuel with the correct 3 blade prop . as far as a more stable boat I do not thimk there would be a difference at all . but thats something you have to feel and judge for your self

But what works well on one boat may not work on another even thou its a very similar boat


as far as fuel burn you say you less fuel but your cursing slower you may have gotten the same results by slow down with your 3 blade or taking out some pitch. may be not ? just something to think about

[ready123]

I say he will get on plane faster and burn less fuel with the correct 3 blade prop . as far as a more stable boat I do not thimk there would be a difference at all.
as far as fuel burn you say you less fuel but your cursing slower you may have gotten the same results by slow down with your 3 blade or taking out some pitch. may be not ? just something to think about

Huh... it is a known fact that 4 blades get boats on plane quicker, the extra aft lift also makes that planing speed lower than 3 blade. We are talking about being on plane here not any specific inwater speed. 4 blades are able to transfer torque to the water faster than a 3 blade thus the quicker to plane time. This really is independant of boat.... the correct 4 blade for your F36 will exhibit these differences to the correct 3 blade... just as it does for my F32.
In the hopes of getting you to agree with that fact, maybe you should go back to that book or do a google search on the web. Anything I have seen there tends to agree with my argument.

Your last point of slowing down the speed with 3 blades seems to ignore that with boats (high load application much like tractor trailers not cars) the rpm and engine's related power curve is of more importance.
My engine was at same speed for both props... the boat speed was less with 4, the fuel consumed per mile was also less. Yes the time to get from A to B went up but my $ spent went down and that was more important for me
Also your statement of taking out some pitch on the 3 blade confuses me. I'm a believer in the theory that there is only one prop of any one design (3 or 4 blade) that is ideal for each boat (displacement) and engine power/transmission ratio mix.

[prowlersfish]

Michael , where do you find your facts ? "Huh... it is a known fact that 4 blades get boats on plane quicker" Fact ??? I do not think so some times that maybe true but sometimes it is not not .

You say you lost speed but saved fuel . I said you would have gotten the same results by slowing down with the 3 blade . I still say that sure the rpm would go down a liltle so what ?


it takes X hp to move your boat at say 18 knots be it at 3200 rpm with a 4 blade or 3000 with a 3 blade rember your not useing the max hp at those rpms so the power use would be about the same and so would the fuel burn .

as far as only one right prop for a boat . I think how you use it has a very big effect what works for you may not work for me .

4 blades do what you want then to on your boat and thats all that matters .
I am happier with 3 blades for my use and boat/drive train set up . a few knots matter to me if I want to save fuel I back off.

i

[ready123]

Michael , where do you find your facts ? "Huh... it is a known fact that 4 blades get boats on plane quicker" Fact ??? I do not think so some times that maybe true but sometimes it is not not .
You say you lost speed but saved fuel . I said you would have gotten the same results by slowing down with the 3 blade . I still say that sure the rpm would go down a liltle so what ?
it takes X hp to move your boat at say 18 knots be it at 3200 rpm with a 4 blade or 3000 with a 3 blade rember your not useing the max hp at those rpms so the power use would be about the same and so would the fuel burn .

I disagree with the conclusion of fuel burn and that hp=speed, hp=rpm I will buy. Anyway how can we decide anything based on pulling example numbers out of the air like you have done. They are meaningless... we could use any other number combination and be equally wrong.
What about 4 barrels in or not in the fuel burn equation?
The theory also suggests that in mid range speed a 4 blade will give more boat speed than 3 blade due to less prop slip...
The 4 blade gives boat more lift (fact, check the science) so it's drag in the water may be less than same boat speed of 3 blade and so in fact require less HP.
Facts are from propeller design engineering,,, do your research. Just like the science says that 4 blades are more efficient at mid range speed and power curves. Also all your reasoning ignores the change in boat attitude/drag with different in water speeds and prop lift.
Your conclusions are too simplistic... it is not simply speed and revs... this is about pushing a weighted boat thru the water and many other factors come into it.
We will have to agree to disagree... I am unchanged in my view of the advantages of the 4 blade vs 3 blade.
The only real way to convince you would be to do a test ONE boat with both ideal props and see the results. I have run both types on mine and have chosen the 4 blade.... it is unfortunate that I did not take more readings with the 3 blade... most of the test numbers I have are with 4 blade.
I'll retest 4 blade and no dinghy this year to see how much that pushes my cruise speed up... all my data with 3 blade was no dinghy and about 9 hrs of running before she came out for the winter.
Since then I have done 150 hrs with 4 blades. 0.9mpg overall pretty close to the Trojan spec for 3000rpm and 21.3mph of .95mpg and those engines are 1977 era.
The original Trojan spec sheet sheet that came with my boat says 3 blades were spares, so Trojan put the 4's on by preference.