Re: Confused and second guessing myself
The tail was probably added to help the hole shot. They are ok at lower speeds but can cost you if you are a high perf nut which you said you aren't....so I guess I'd leave it.
You may be trimming your prop out of the water to get the 5600 rpm. The fact that you were loosing speed tells me that.
So, unloading the engine allows the rpm's to increase which tells me that the setup is giving you all you are going to get.
Cutting some pitch out of the prop will raise your rpm's (good) and improve your hole shot (also good). Will normally cost you a little top end mph, but you don't know till you run it. Big guys figure about 200 rpm per inch so an inch or two, going to an 11 or 12 pitch will put your rpms up at the top (where they say they belong) and help your hole which is what you want.
Only easy way to figure the gear ratio, if you can't find it anywhere else, is to mark the prop and the flywheel so you can measure revolutions. Pull the plugs to make it easy to turn (by hand) and spin the flywheel and count your prop revolutions in Forward gear. The gear ratio will be a reduction so the flywheel will rotate 2 revs for the prop 1 rev if 2:1. If less than 2:1 flywheel will only turn 1.75 revs (if 1.75:1 ) for one complete rotation of the prop.
I just looked in a '94 Merc sales brochure I have and their 40 hp uses a 2:1 gearbox and their 50 uses a 1.64. So you should be in that range somewhere.
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FYI, one of the reasons for the big change in their engines is that the 40 is the old 4 cyl design of 30 years or so and the 50 is a 3 cyl of current design.
The 3 cyl is running lots more cubic inches, 3 rather than 2 carbs, and develops 25% more hp on the same sized lower unit, so it has more torque and can turn the same size prop faster.
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I am running a very similar boat to yours, same length, alum, with a 90 on it if that gives you any idea as to where you are in the "how much hp is enough" equation. The picture in my Avatar is what it does at WOT. That pic is taken at 50 mph with my cell phone camera.
Getting back to what the orig owner said may be true. I found on my boat that at around 20-25 mph I have a lot of boat in the water and a lot of drag. Once I get up and going the boat planes off and "gets out of the water"....much more efficient. This would be the right answer as if you lightened the load on your engine, your rpm's would go up and you could get to that speed he mentioned from your existing prop.
Like I mentioned above, I have twice your hp and I'm getting more than twice the speed. That's not normally how it works. Higher speed usually comes at a price and the higher it is the higher the price.....but when you get the boat going fast enough, you get the hull out of the water and that cuts your drag so it takes less to get more and that is what is happening in my case......your slippage goes down too and that's free power.
In summary, go down an inch or two on your prop and enjoy your boat.
Mark
