Height on Transom

[Kiwi]

My 1992 Suzuki 85hp sits with the cavitation plate level with the bottom of the hull. I get 4900rpm at WOT. The manual states operating range is 4800 - 5500 rpm. I feel that the rpm should at least be just over 5000 to be in the middle of the range or am I worrying about nothing. The motor has a 21" SS prop. I think I read somewhere that when you have a SS prop you can raise the motor higher than you can with an aluminium prop ? If this is correct how much higher could I raise it and would this increase the rpm at all ? Where the motor is set at the moment it never cavitates even on the tightest turns.<br /><br />Thanks<br />Mike

 

[12Footer]

Re: Height on Transom

It's really apples and oranges,Mike. Lighten your boat as much as you can for testing.<br />The RPMs will probably go up to the higher end of the operating range.<br />If not, reduce this pitch of the prop by one size.

 

[Grant Ingle]

Re: Height on Transom

Mike,<br /><br />There are many different variables which can effect the performance of your boat. Is the boat constantly in the water where the hull gets "hair" (algae) on it? You didn't mention but I assume that you have tilt and trim on your motor. Are you trimming it as far as you can? If the boat has its standard load ( fuel, gear and passengers) and you still cannot achieve 5000 at WOT you may want to drop to a lower pitch prop. <br />good luck and happy boating.

 

[Kiwi]

Re: Height on Transom

Thanks 12Footer / Grant. I'm trying to avoid getting another prop. A 19" SS prop for this engine here is NZ$840 so if I can somehow squeeze another 200 rpm to tip it over the 5000 mark then I would be happy. My spare 17” prop takes it up to 5600rpm, so realistically a 19” is what is needed. Just a bit of background on this boat for you and so I can hopefully put this to rest as I have been toying with this for ages. This hull, when it was new, had a 1973 Evinrude 65hp, electric shift – NICE motor ran like a dream and we never had any problems until 1992 when we realised that the head gasket has been letting water into one of the cylinders. No one told us we should have been making sure the head bolts were at the right torque ! M’Dad was persuaded by the dealer into buying an 85hp Suzuki instead of repairing the Evinrude Anyways the Suzuki is a little too heavy for this boat, it does not handle as well as it did with the Evinrude, also it was setup with the wrong prop in my eyes right from the beginning. One of those dealers who once they got their money they don’t want to know you. The boat is 15’6” made of fiberglass and has a deep V.<br /><br />Is it worth trying to raise the motor on the transom by one 'hole' ? Would that increase the rpm's ? - I have a feeling it might not as the prop is still taking the same size bite in the water. Yes, the motor does have trim/tilt. I can tell by the feel of the boat and the appearance of the wake when the engine is trimmed about right, but its a very fine line between having it trimmed correctly and feels right to when it starts to 'porpoise' <br /><br />Any suggestions ? Or do I just go and buy a 19” prop and be done with it ! LOL ! One more thing, is there a big difference in running a SS compared to an aluminum ? An aluminum 19” I can get for NZ$295<br /><br />Cheers