Engine performance and prop interaction

[zpbruno]

I have a newly acquired 1973 Bonito tri-hull with a 65 HP 1973 Evinrude. I've been going through fixing this and that and am wondering if I am finished with engine repair and need to move on to propeller tweaks.

The boat is fiberglass, 16 feet long, I don't know the weight but I see other similar vintage similar hulls come in at 1000 pounds plus or minus. WOT for the motor is rated at 5000 RPM, and it has what looks like the stock Evinrude prop for that motor of 13.25X17 that has a few minor dents. I confirmed today that there was no hub slippage after about 45 minutes of cruising at pretty much max speed for this combination..

This afternoon I took it out on calm water, with about 320 pounds of passenger load. With the throttle lever pushed all the way forward, which I can confirm yields horizontal throttle plates, the boat runs about 25 mph by GPS with about 4600 RPM from a cheap inductive tachometer.

I'm new to boat ownership but it seems like this combination should have a little more to give in speed. My questions are inter-related:
1) Does the fact that I am getting so close to rated 5000 rpm say that the engine is perhaps in decent working order and I can stop looking for problems there?
2) Do I adjust the prop pitch now to try and hit 5000 RPM? If so, what direction do I go?
3) Should the 5000 RPM be happening with the throttle lever pushed all the way forward, or should I be shooting for 5000 at, say, 3/4 lever position?
4) Any wild guesses on what this boat and motor should be capable of for speed?

 

[Scott Danforth]

tri-hulls are heavy to begin with and from a hull design, they are not spectacular

being a 50 year old boat, if you have any rot or water in the flotation foam its going to weigh even more. you need to look up the specs for your boat and you need to actually weigh your boat.

your 1973 outboard was rated at the flywheel, which means you have about 58hp at the prop.

you shoot for what ever the publishes WOT RPM is when you have your boat loaded and the throttle mashed against the stop

your boat would do 65mph on the highway on a trailer behind your tow vehicle. other than that, not knowing the actual weight and condition of your hull, or the condition of your motor, its a guess.

 

[Chris1956]

Boat speed will be prop RPM times pitch. This gives inches per minute. Now adjust that to miles per hour, minus slip percentage, to give boat speed..

There are prop calculators which will give boat speed based on engine RPM, gear ratio, prop pitch and slip. You could punch in your numbers and see what the prop slip is. If too high, you can maybe do something to lower it, which should increase speed.

Since the motor will usually turn the max RPM it has the power for, to achieve greater RPM, pitch of the prop is lowered.

Normally outboards have a RPM range (say 4000-5000) where power is maximized. You want to be in this power band at full throttle, with a normal load.

I would expect that boat/motor to be capable of 27-32 MPH. But that is just an educated guess. Try the prop calculator and see what it thinks.

 

[Texasmark]

https://www.go-fast.com/Knowledge-base/Find-your-prop-slip/Prop-slip-calculator

Gear ratio is just the number, like 2.41 comes to mind for that engine.
Pitch obvious
RPM obvious
Punch the theoretical max.

Plug in current speed and solve for slip. 10 or less would be a good number. Back in the day, 55-60 hp OMC Loop Charged engines, no Power Trim, on 15-16'' trihulls, many manufacturers/owners ran 15-17" pitched props, my boating circle having such, and could pull a single skier on w. skis and have a couple of folks in the boat. Speeds, don't remember, not stellar, but fast enough to have a good time and reasonable "get up" times for the skier...lighter ones.

Popular trihulls of that size usually were reasonably flat at the stern, slight "dead rise" (which makes planing out easier than a deep V) and were make from a layer or two of fiberglass woven roving and then chopper gun sprayed in glass particles as compared to moderately deep Vs with significant dead rise at the stern, with mainly hand laid roving in numerous layers....making for heavy boats....but they rode well in a chop as compared to getting beat to death on the former....that's why my first boat listed below, lasted the season...barely.

First trihull was a '71 16' Chrysler Sport Fury and it weighed more like 600# shallow monohull which made for easy planing but very rough riding on the water. That lasted a year and I bought the deep V style and ride was greatly improved, course the HP changed from 55 to 125 and length 2' increase.

Your expectation of 27-32 I think is realistic for the boat if of the shallow V design and you are boating alone with the 17P prop and it probably is the OEM supplied prop for that engine. You are right at the top of the recommended RPM range which is where you want to be with your regular load. If you prop higher to squeeze a few more MPH out of the rig, when you load it up or want to play with water toys the engine will fall on it's face. BTDT

I'd replace what you currently have!