Initial Prop Selection

[Skyewinder]

I am looking for advice for a starting point in propeller selection. I just mounted at 200 h.p., Mercury EFI on my 17" tunnel hull. The boat weighs 500#, fuel is another 100, the engine is 400, and a operator is about 200#. The engine has a 7" jack-plate set back. I am currently running a 4 blade, 15.25" dia., 24P spare prop from my Powerquest just to get the boat on the water. At this time the tachometer is not hooked up, but will be very shortly. I would appreciate any advice as to where would be a reasonable beginning point in prop selection. Thank you very much!

[engineermike]

What is max rpm and gear ratio?

[FrenchPhil]

I'd start with a 3 blade 14-14.5" x 24-25" cleaver. What's your gearcase type & engine height ?

[Skyewinder]

The maximum rpm is 5800 and the gear ration is 1.87. The motor is set up with the cavitation plate even with the bottom of the tunnel. Thanks

[Skyewinder]

The gear case is standard factory with a 1.87 ratio and 5800 rpms. The outboard's cavitation plate is even with the bottom of the tunnel. Thank you for your prop suggestion.

[engineermike]

I’m not sure if I believe this but a 1200 lb total boat weight with 200 hp is supposed to go 110 if it’s an efficient cat. At 5800 and 6% slip with a 1.87 ratio you would need a 40” prop to get there. Hummm, doesn’t sound right at all. I’m thimking start with a 30” 3-blade cleaver and work your way up. It should do 80 with that.

[Skyewinder]

Hi Engineermike,
The boat was built out of marine plywood, with the top skin of 1/8" plywood, and the bottom tunnel is 3/8" plywood all the way around with reinforced 3/4" and 1" mahogany supports. The bottom only has been fiberglassed. I know the weight sounds "fishy" but it's pretty accurate. Thanks for questioning the weight, and for your prop recommendation.

[engineermike]

Oh I wasn’t questioning your weight....just that I haven’t heard of many boats going 110 mph on only 200 hp.

[FrenchPhil]

What speed you got so far ? If the cav plate is lined up with the sponsons, the engine is way too low.

[HStream1]

There are a few issues here that hasnt been addressed. (1) Hes got a Fishing Lower on a tunnel? (2) He cant raise it but so high or hell loose water pressure. And (3) He cant be using a surface piercing Prop design on a boat with a lower thats buried. He needs a through hub prop vs over the hub. He really needs a LWPU lower to get the maximum efficiency out of the Tunnel and motor raised up so the Prop shaft is even with the sponsons as a starting point.. The other unknown is what length mid is currently on the motor? 20", 15" 12"?? and what does the tunnel really need as far the proper mid length? Most small tunnels i.e. 17' wont like a 20" mid. And if there is enough vertical adjustment to get the motor up. Then hes thrown the boats geometry off by being Top heavy. Just my 02.