Sea trials-one major problem

Discussion in 'Powerboats' started by Guest, Oct 13, 2002.

  1. Guest

    Guest Guest

    I'm hoping for some sage advice here...I signed on to a project in progress, we took a 28.5 John Allmand crusier, gutted it to bare hull shell and built it up from the stringers. What used to be a cabin style boat is now basically an express with a very small cuddy, mostly open deck, with aluminum 1/2 tower and hardtop. We installed a centrally located Johnson and Towers 671 TI, rated (at very high rpm) 460 hp. Gear is a Twin Disc 407 1.5/1, prop is 19" pitch X 24" dia. The boat looks great, the fishing layout is fantastic. We were fairly careful to not make it too heavy, by necessity the stringers and bulkheads were beefy but decks are 1/2 ply/glass, and the boat is quite a bit lighter than stock with twin gas engines. It floated about 2" above the lines when launched.

    Ah, but here's the problem. It squats, bigtime. The engine is mounted at a 15 degree angle since the gear has no down angle that was the only way to make it work. Looking at it, the 15 deg shaft angle doesn't look excessive but adding another 8-10 degrees of running trim makes me concerned that the engine is now at too steep an angle. I'm kind of surprised, I had worried about a bow down trim problem as the engine is quite a lot further forward than the original twin gas installation. It runs up to about 20knots, acts like it wants to break over and plane but it doesn't happen.

    So, a little calculation shows that at 1000 shaft rpm with this prop should give around 18knots or so, meaning we're showing slippage of 40% at least. I'm thinking the problem is insufficient planing surface, the boat is plowing all the time and can't get on top. Comparing to a 31 bertram, this hull is a couple feet shorter, about the same beam, much less v. I notice the 31 has reverse chines protruding from the sides of the hull, as I recall they're about 3" wide or so, they have a concave surface facing down.

    If I'm on the right track, I see a couple of options, add the chines, add huge trim planes, or both. I'd like to see more speed, and a more level running attitude. The estimated weight of the hull as it stands is 3500 for the hull, 2700 engine, gear, shaft, 1500 fuel, 1200 occupants and gear. Seems like 400+ hp ought to move it along right smartly. Would appreciate your thoughts and advice, three years of weekends are at stake here! I'lll post some photos within the next couple days.

    Thanks, Chuck
  2. Corpus Skipper
    Joined: Oct 2003
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    Location: Corpus Christi TX

    Corpus Skipper Hopeless Boataholic

    One thing to consider here. With the engine so far foreward all the weight is bearing down foreward of the theoretical axis the boat wants to plane over on. is there any room for adjustment of the engine position? I realize you can't move it much due to shaft alignment, but even an inch or two may help. Moving my fuel tanks 2" foreward resulted in a much better running angle (my boat is back heavy). Try weighting down the aft end of the boat with a barrel full of water and expirament with the placement to change the run angle. Your hull should plane around 13-14 knots, doing 20 and still plowing must be a weight distribution problem. Hope this helps!
  3. badges65
    Joined: Dec 2002
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    Location: New Zealand

    badges65 Junior Member

    are you saying you are running bow high??
    I hope you have a correct mounted oil pump pick up for that engine!!
    did your oil pressure change when you got it up to speed??
    twin disc 407?? you sure ??

  4. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Was the original installation with outdrives?
  5. Guest

    Guest Guest

    Correcting for boat squat

    Does the boat sit on her lines when still in the water?
    If so, your weight distribution is probably correct. You may want to check that the shaft angle intersects the Center of Gravity (CG) or Center of Flotation (CF) before shifting weights. When the shaft line passes through the Center, it drives the vessel straight forward on the boat's lines. If it passes below, the thrust of the propellor lifts the bow of the vessel which, in moderation, is an asset in a planing hull. With the angle is above the Center, the opposite occurs. Check this first. Only then, consider moving weights around.
  6. trouty

    trouty Guest

    If it's

    not climbing it's own planing wedge and your calculating over 40% prop slip - wouldn't you want to do a combination of either/ or two things.

    1. either get a prop with either more diameter if space allows or more blade surface area (4 blades?) to improve slip rate and see in the extra power translated allows the engine to still rev to manufacturers specified WOT rev range and also whether that additional forward momentum allows it to climb it's own planing wedge?

    If more prop surface area reduces WOT rpms then you might have to drop pitch to achieve what your looking for with less slip.

    2. Alternately adding more planing surface might help.

    I'm no expert but thats a rough guess at it.


  7. Diegof
    Joined: May 2004
    Posts: 2
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    Location: Panama

    Diegof New Member

    Allmand 28

    I just bougth that very same boat, but I am thinking of lossing the twin inboards, lowering the aft deck and installing a pair of outboard mounted on a bracket. Any comments would be appreciated

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