motor height help

[mosesonwater]

my little buddy and i are putting an old outboard on our nice old granprix tunnel boat, but were not too sure of the correct height of the propeller in relation to the hull. all the pictures we see look like the the mid line of the prop shaft is about even with the bottom of the sponsons . we don't have the luxury of a adjustable jack plate, and we're not gonna race any body , but we do want to cruise the the glass when we have it. we tried putin the cavitation plate even with the bottom of the tunnel, but when we take off the prop cavitates, so its too high now, is there a rule of thumb, or just keep going down till she grabs nice at speed and doesn't cavitate. thanks for any wisdom you guys can provide for us to have fun with the nice old gran prix. moseson the water cavitating.

[pat]

Your assumptions about prop height are pretty accurate assuming your engine has a racing gearcase and prop. What is your outboard, horsepower and prop? Cavitation can be a function of being too high but it can also be a function of too low of a pitch especially if you don't have power trim to help you plane out. Make sure you are getting enough water cooling no mater what height you are attempting to run. A tach would tell you a lot about proper set-up.

[mosesonwater]

pat; thank-you for helpin out. i'll give you a good laugh and some more info. we've got a 1985 johnson 15 hp with a stock lower unit and a 9 1/2x 10 prop. not much of anything. i measured and it looks like we gotta go down about 3 1/2 inches to get the mid of the shaft even with the bottom, and probably should go more with the stock lower unit to keep water so we don't fry.we're way under powered i know, but its all we got right now, and even with the 15 this nice old boat will give us chickens a thrill if i get her in enough. she takes off like a bullet even with old 280lbs of me in there, but as soon as she raises up she cavs. we'll lower her down by puttin 2 channels and a lower piece of transom wood on the back and see what happens till we can afford real hp. i'll try to get you guys a nice picture in the mean time. thanks again

[pat]

I agree. You need to go down,down,down to gulp water and to reduce cavitation. If you can, I suggest that you build a 2" or 3" set-back on the transom (kind of like a jack plate) to lower this particular engine. Use a 2" aluminum U bracket ( 2" x 2" channel cut in half and two 3/4" pieces of plywood laminated together as a motor board) set somewhat lower than the orginal transom. What this will do are two things. 1) It will allow you to pick-up more water without having to go down as much if you were mounted directly onto the transom and 2) It will give you lift which is really what you want with a tunnel. Unfortunately, with a 15 HP you will not generate enough speed to compress any air (lift) inside the tunnel so you will be more "wet" than you really want. If you want to stay "stock" I would look for a used 50HP 4-cylinder Mercury (short shaft) and a 15" 2-blade cupped bronze prop or something similar. This engine will likely get you 45 to 50 mph and you will be generating lift through tunnel areodynamics. Alternateively you could look for a Sport "C" engine Yamaha or Tohatsu. That will put you in a 50+ mph bracket.

[mosesonwater]

motorheight

i'm on the same track pat. if i add the c channel and new lower transom , then she'll go down nice. we had a nephew give us an 88 merc 4cyl 80hp, but its too big, so we're gonna trade somebody for like you said. too bad though cause this one has power tilt also, but too much hp and weight. the enclosed pics show our little project. the past owner had the transom redone . its 16'' to the tunnel from the top, and it's 22'' from the top to the lowest part of the back of the boat. we have the top cover also, but its in the garage. we got the md dnr to give us a title to get numbers because we certified thru the old owner that it had never been titled before , only raced, and they fixed us up to be legal on the water.if got another motor, would the height of this new built transom still be suitable for a short shaft[16''], or should we look for something in the 21, or 22 range?

[pat]

Suggest you stay with a 16" to 17" transom (short shaft). You need to keep the Center of Gravity as low as possible. Good luck.

[mosesonwater]

moses motor height

thanks pat , i ordered the c- channel and can't wait to try it out. we'll let you know whats happenin after we get it going on

[david bryan]

you need a biger motor that 15 hp will not push it over 20 mph

[mosesonwater]

we're working on it bryan, got an 80 with power tilt we're working on to trade for a 50 to get us up to speed, but as a newbie it's been fun playing with the 15, just to get in the water and cruise around alittle.