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Old 02 April 2007, 07:30   #1
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Tramsom height/shaft length

My northcraft transom is 22" the outboard that is on at the moment has a 20" leg to cavitation plate. The cavitation plate sits 2" above the lowest point on the hull the planning pad. Does this seem right? Boat dirves ok with no problems. I am looking at a new engine longshaft 20" just want to check before buying.
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Old 02 April 2007, 15:00   #2
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In theory you should have a 25" shaft raised up on the transom by using different sets of holes on the saddle. Some people recommend having the cav plate above the hull or even part of the prop above it so it's not set in stone. A lot depends on the way the water exits from under the hull, this can vary with the weight of the load on the boat and trim angle of the boat.

If the 20" works without any cavitation or prop slip in any conditions under accelaration or tight cornering and allows you to trim up to a reasonable level then stick with 20" I guess. Any problems then go to 25", the worst can happen is you lose a knot or two off the top end.
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Old 02 April 2007, 16:07   #3
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Cav plate hight

Quote:
Originally Posted by ALANT View Post
My northcraft transom is 22" the outboard that is on at the moment has a 20" leg to cavitation plate. The cavitation plate sits 2" above the lowest point on the hull the planning pad. Does this seem right? Boat dirves ok with no problems. I am looking at a new engine longshaft 20" just want to check before buying.
Hi Alant,
Rule of thumb is cav plate level with the boats keel & a slight offset to starboard pending on Hp, however there is a lot of variable's here ie; new engine's Hp, driving style (top speed overall or low down grunt required), hull design & new prop size/type. So probably better off talking to Northcraft & the new engine sales team as you dont want to end up getting it wrong, drilling lots of new holes & having to fill em back in!
Regards Mike.
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Old 05 April 2007, 16:21   #4
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You need a longshaft. Extra long will be too long

If you get a bit of slip (unlikely), then you could reduce the transom height by lopping an inch off the top, but if the boat drives ok with your current motor, it should be fine with the new one. Before fitting it, If I were you, I'd have a word with John at Northcraft to see if you need more offset for the new, more powerful motor.
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Old 08 April 2007, 13:18   #5
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Quote:
Originally Posted by ALANT View Post
My northcraft transom is 22" the outboard that is on at the moment has a 20" leg to cavitation plate. The cavitation plate sits 2" above the lowest point on the hull the planning pad. Does this seem right? Boat dirves ok with no problems. I am looking at a new engine longshaft 20" just want to check before buying.
A lot depends on which engine you have/want. I've had 3 Northcrafts, and the second one had to have the engine raised to about your position, otherwise it wouldnt rev correctly and drank fuel. It made a hell of a difference. Convention does indeed suggest that the cav plate is level with the planing pad, not so for merc or mariner, only 2/3rds of the prop needs to be in the water for best results. You will get more slip, particularly from hole shot, but you wont need to visit your petrol pump attendant so often and top end speed will be higher.
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