35 horse prop?

[chipeason]

Took my 1988 Blue Fin aluminum boat out after a complete restore and was very disapointed in the performance. 35 h.p force motor, 15' aluminum boat, 4 gallons of gas, 2 -180 pound men and a 12 pack of beer. Top speed 23 mph (GPS) passenger had to get in front of boat to plane out is there a problem?
1988 force 35 HP
15' aluminum boat
weight of boat unknown
5000 RPM
23 MPH (GPS)
10-3/8 x 12 aluminum prop

Thanks for any suggestions!!

 

[pootnic]

Re: 35 horse prop?

When you say "complete restore" did you add alot of weight(wood),the hull in good shape?
It does seem slow and the rpms seem alittle on the lower side as well,with a 12" prop.

 

[steelespike]

Re: 35 horse prop?

Prop calculator says 19% slip thats alot. Tell us more about the boat.
Is the prop in excellent shape?
At a pretty normal slip of 6% speed would be about 26.
A heavy boat can create slip a poor or poorly suited prop also.
Is the prop 12 or 12 1/2"?
The force isn't noted for its speed. Prop calculator indicates
At 5250 with a 12" prop it would make about 28
Is the bottom clean and smooth?
Did you try different trim settings?
Is the Antivent plate(just above the prop) about even with or slightly above the bottom?

 

[chipeason]

Re: 35 horse prop?

I replaced all of the wood flooring in the boat. The only added weight that comes to mind is that I used 3/4 " plywood and not 1/2". The hull is clean and smooth. The antivent plate is level with the bottom of the boat. I did try different trim settings, all the way down to 2nd and 3rd hole. Passenger had to move up front to plane out, after plane on 3rd hole noticable porposing was observed. The propeller is in good visiable shape and 10-2/8 x 12 is stamped on it. Wonder if the prop is spinning on the hub? I will check that. I know it is a 35 horse turd but you would think it would at least plane out on its own, I dont expect 45mph but 30 would be nice. Thanks for the help!!

 

[steelespike]

Re: 35 horse prop?

Is this a tiller setup?Try moving the gastank and battery if any forward.
Usually a spun hub is a total loss of any meaninmgful thrust.
Check that the throttle opens all the way and it picks up the carb at the right point.
What is the compression.
The 28 mph I quoted would be on a light fast boat.
Porpoising indicates Too much trim,perhaps too much weight in the stern.
If 2nd notch isn't enough and 3 is too much you can buy or make wedges to increase notch 2 about 1/2 a notch
or reversed to subtract 1/2 notch from the 3rd notch
You might see if a prop shop can put a little cup into the prop.
Explain it is slow to plane and may have high slip.Be sure they under stand you need better hole shot
the cup for that should help the slip.

 

[Frank Acampora]

Re: 35 horse prop?



If the engine is also around 1988, it is not a turd, but it is also not a powerhouse. Later Force 40 hp engines did use larger bore and stroke and did perform way better. Simply put, if the 35 is working correctly--good compression, good spark, and good fuel supply---then you need a bigger engine.

For your 35, with the current load, 12 inch pitch MAY be overpropped. 10 1/2 to 11 pitch should get the engine up to 5500 rpm and performance may be better. Those stock pin drive props are regressive with very thick blades. An aftermarket Michigan will perform better. Again, if your engine is around 1988 these props are the same for Chrysler and Force engines all the way from 1966 to 1990s. 35, 45, 50, and 55 HP engines all used the same props. You can pick up these "pin drive" props cheaply on the auctions. Here is a bronze one I bought for 12 bucks.

 

[steelespike]

Re: 35 horse prop?

Frank should be able to help you get your Force setup.
Frank; If you feel he is overpropped you must feel his reported rpm(5,000) is not accurate.
That may be so as the prop calculator says 19% slip.Perhaps the rpm is actually less?
In his specs list he says the motor is an 88, my limited rpm info says 5250.Is 5500 considered
a reasonable max or can they go higher?

 

[chipeason]

Re: 35 horse prop?

Thanks for all of the suggestions! I have been doing some reading and I think I will try a set of "smart tabs". Have you guys used them?

 

[steelespike]

Re: 35 horse prop?

I haven't.They can work well if selected properly and installed right.
Though I do think the tabs are better and don't know what the Tabs cost under the circumstances you might consider
a foil/whaletale /doelfin for the money might do well. You just need to be sure the antivent plate
is close to the surface on plane. Both can cause poor ride and erratic handling if set up wrong.

 

[Frank Acampora]

Re: 35 horse prop?

Quite frankly ( no pun indended) I do not remember the exact specified RPM on the smaller engines. However, 5500 as a top RPM is not excessive. The same engine with larger bore and stroke (Chrysler 55, Force 50 -3.3187 X 2.80) Is rated at 55 HP at 5500 RPM. I believe the Chrysler 35 (and 45) is rated at its horsepower at 5250 but range is 5000-5500. 6000 or more is not out of range with these two cycle engines but after you reach the peak HP range, it starts to drop and if you go too high, you will again lose performance. So really, 5500 is about as high as you would want to go in a recreational engine.

At any rate, just as a semi-comparison, my friend had an older Rude 28 on a 15 footer and it was a dog--he could not believe it. I had him drop the prop to --I forget-- one or two inches less pitch and you would not believe the improvement. It was like a completely different engine and top speed actually went up. Now, he did not have a tach and I can not say what the RPM were with either prop but it sounded OK--not like it was bogging down or over-revving.

He was at first sceptical, thinking that a lower pitch would decrease speed and performance but after trying, he was a believer.

Some of these little engines are sensitive to prop size and if overpropped, the slip will go way up.

Personally, I have nothing against smart tabs, but first I would try a prop change. These props are available on the auctions relatively inexpensively.

 

[steelespike]

Re: 35 horse prop?

Frank is the guy on these Chrysler/Force motors.
If you wanted to try a prop with newer technology there are 10" and 11 1/2" prop here at iboats.
They are pricey but at least you know the prop is new and undamaged. Frank seemed to think
a 10" would work well.

 

[chipeason]

Re: 35 horse prop?

Man the smart tabs are amazing!! Completely cured the getting on plane problem. Same load same location on boat and passenger did not have to get in the front to plane out. Top speed is the 1 mph more. I put a 10 pitch brass prop on it but the weather has not allowed me to try it. I will post results after trial.

 

[steelespike]

Re: 35 horse prop?

Thats great to here.The Smart Tabs can be a big help when installed correctly.
I've seen videos where the boats were forced to lay down real flat and handle strangely most likely from poor tab setup.
My only beef of adding tabs is very likely the boat still has a stern weight bias.