Outboard brackets

[Guest]

I have just purchased a 22'8" Mako centre console open boat with a T-top. It is powered by two 90hp Mercury outboard engines on individual aluminimum brackets which set the engines back about 28" from the transom.

The engines appear to be positioned too low. When travelling at slow speeds or stopping the water in the wake is nearly up to the bottom of the cover joint. I have had the misfortune to lose one powerhead due to connecting rod breaking and coming through the crankcase in four places. My wallet hurt.

I have now raised the engines as far as I possible.

Can anyone advise how to properly set the engine cavatation plate in relation to the base of the transom. Eg should it be level or 1 or more inches (We are still using imperial measurements in St. Kitts - former far flung British colony in the Caribbean) above the transom.

Thanks Doug

 

[Guest]

I was told that it should be level with the bottom if at all possible. That would seem to make some sense as it would lay on the boundary between the fast moving water under the boat and the disturbed water behind the transom. Mind you I wouldn't be surprised if some clever sod has a very good hydrdynamic reason for doing something else.

 

[Guest]

When I used to race boats way back in the 1960s my boats were prepared by Sergio Carniti. Carniti's factory always set the cavitation plate 1/2 inch above because they reckoned this gave maximum performance.

 

[Guest]

You can get hydraulic outboard brackets which will raise/lower the height of the engines, mainly intended for the bigger engine options.

The height of the outboard in relation to the bottom of the hull, the cav plate can be some considrable height above the bottom of the hull, as much as 3+ inches.

However, on a standard outboard such as yours you will be limited by the water intake depth, but different bottom end gear caseing designs compensate for this, Merc do have an extensive range, again intended for the higher HP bracket.

The higher the prop rides out of the water the corser the blade pitch you can run, the higher the performance will be. However at this end of the performance range the results are at lower to mid engine/hull speeds the propeller will cavitate. When the speed increases the corse pitch prop will ride half in/half out of the water, hence the larger ploom of water behind, at this point it will bite correctly, and the knots just pile on. Although its worth mentioning these are not your average propeller designs.

No dout you have guessed this is all down to the hydrdynamics of the theory of the less there is in the water, the less the drag will become. Its how all race boats run.

Your problem will be reduced by installing leg extensions of and 1" or so between the mid section and lower gear casing. So you can run the cav plate at a suitable height for your installation/HP/boat type but the topend/engine will by higher out of the water.

Happy days..

 

[Guest]

Every 6" back from the transom the motors need to go up about 1.5" and thats with std props. do your brackets provide any bouyancy? tubular type brackets are killers at rest in swell. you could put 25" shafts onto the motors to raise the powerheads but i would go with ditching your brackets and making an aluminium marlin board come motor bracket that goes right down to the planing surface and full width of the boat. if you need part nos. for ext kits or anything let me know. good luck doug!

 

[Guest]

Andy

The brackets are Armstrong aluminium that position engines 29" astern of the transom.

I have had feed back from another source which suggests 1" rise for every 10" back. On this basis mine should be approximately 3" up. I have done this (3 1/4") and the engines are on the verge of cavatation. If I used your 1 1/2 I think I would be too high.

I am considering filling the brackets with two part sealed cell foam. I recon this would give 2.5x1.5x 0.75 =2.81 CF per bracket x 2 x 62lb/CFwater = 348.75 lbs of bouyancy. I have a panel forming a platform across the brackets at present.

I am interested in getting details including costs for your suggested Marlin board come motor brackets.

I was considering getting trim tabs.

Would appreciate your input.

Thanks

 

[Guest]

I come from the old scool and dont really believe brackets are any good with hulls running sub 60mph. My thinking is not just aimed at bouyancy but extending the running surface of the hull right under where your bracket is now. Then its just like having the motors right on the transom, no need for xl legs or good props, engine height would be normal with vent plate level or a poofteenth below. If at 3 1/4 you get ventilation then try a pair of trophy plus props, youll probably go higher with them! However, better to go with a "filled in" type of extension. I cant suggest any cost that would mean much, my local guy would charge about $3000 aus for a good one with nice steps etc. Does any of that make sense????? regards Andy