40hp 2 stroke Yamaha and 16' aluminum Starcraft

[ChrisMA]

I need a reality check for my boat motor combination since I am not sure the tach is reading properly. This is my first 2 stroke outboard so I am not to familiar with its operation. I have 16' aluminum starcraft with 40hp 2 stroke Yamaha model 1989 6H4 L 558276 - 40etlf. At warm idle I am showing around 600 rpm and at WOT I am going 30 mph at 2600 rpm. The RPM seems low based on the engine sound and given my prop 11.25 x 14. I know I am over propped based on the michigan wheel website, but dropping down to a 11.75 x 12 prop like they suggest will not give me a 2000 rpm increase. Does anyone else have experience with a 16' Starcraft and can tell me if the top speed and RPM seem reasonable with my current prop?


Thanks in advance!

Chris

 

[Paul Moir]

Re: 40hp 2 stroke Yamaha and 16' aluminum Starcraft

It's really impossible to be going 30mph with a 14" prop at 2600 RPM. Even with an ideal boat (no resistance) all that would be possible is 18mph.
So something is wrong. Either your speed, the prop pitch (unlikely!), or the tachometer. It's probably the tach. Check the back and make sure the switch is set to 3 (6 poles).

 

[ChrisMA]

Re: 40hp 2 stroke Yamaha and 16' aluminum Starcraft

Paul:

OK, I just checked the tach and it was set to 6p, it sounds like it should be set to 3P. This would give me a 1200 rpm cold idle and 5200 WOT at 30mph (GPS verified)

I would like to try a stainless steel prop, which one should I try? I guess I can go with a lower pitch to raise the RPMs slightly.

Chris

 

[Paul Moir]

Re: 40hp 2 stroke Yamaha and 16' aluminum Starcraft

Sounds like it. 6p is a very common setting for other brands of outboards and larger Yamaha ones. I'm not really big on propellers but I might be able to help out a bit from what I've learnt from walleyehed and Dhadley.

You're WOT range is 4500-5500 and your gear ratio is 1.85:1 right? If that's all right, the theoretical speed is 37mph, so your slip ratio is close to 20%. Slip is caused by drag, so unless your boat is heavy for the HP driving it, that indicates you probably have improvements to make to the setup of your boat that will improve top speed and accelleration without making a prop change. These improvements normally make the engine spin faster so hold off on the prop change until you're sure you need to and you know what you need!

Does your engine have Tilt/Trim? If not, have you experimented with moving the trim pin in and out? Where is the anti-ventilation (cavitation) plate in relation to the bottom of the hull? Perhaps the outboard can stand being raised up higher on the transom some. If you do have Tilt/Trim, can you trim all the way out at full throttle without the propeller loosing it's grip on the water?

If none of that works out, you might try relocating weight for better planing trim. Experiment with sandbags or whatever else is heavy and convenient.

Forget what you've read on the MW propfinder. That is very general information. You can get very specific information on your particular boat and engine setup using your existing propeller which will help you choose your new one. If you decide in the end you even need a new one.

 

[Silvertip]

Re: 40hp 2 stroke Yamaha and 16' aluminum Starcraft

You might even try the Yamaha web site (Performance Tests). You can select a motor, boat, and 2S or 4S in most HP classes. The test reports show fuel consumption, top speed, accelerationi, weight as tested, and the prop being used in that test. If I remember correctly, It also indicates engine height. Your idle speed is way to high. That can raise havoc with the lower unit when shifting.

 

[ChrisMA]

Re: 40hp 2 stroke Yamaha and 16' aluminum Starcraft

Silvertip:

Thanks for the suggestion for the performance bulletins, I went to the website and it seems that they suggest a 10 5/8 x 12 prop and should get around 33 mph. I have a 11 1/4 x 14 and am getting 29-30mph. I have to check the RPM again, it sure does not sound like it is turning 1200rpm at idle, it definitely does not "clunk" into gear as if the idle was too high.

Paul:

No idea on the gear ratio. The a/v plate is 1" below the bottom of the boat. I do have power tilt, but not trim. I have experimented with tilting out a little more, but since the boat is pretty back heavy (gas, battery, me) it is much tougher to get on plane. When I get on plane, the bow is pretty darn low, but I cannot adjust the trim while on plane. Next time I go out, if it is smooth enough, I will try trimming it out a little. Problem all my boating previously has been done in larger, heavier fiberglass boats and this one scares me a little bit at WOT. I boat in Cape Cod bay, so it is usually too rough for WOT. I am more concerned about my holeshot and engine damage from over propping.

Chris