Elizabethan Re-Engine

[Varne850]

A friend at work has the following problem can anyone help?
He has and Elizabethan 29 with a 10hp Bukh and a 12" prop which is suffeting from weak compression and need a major overhaul. He has been offered a 20hp Bukh instead. The probem is the propellor clearance. He believes he needs a 14" prop to be able to make full use of the increased hp at full revs but there is only clearance for 13" Three questions.
1. Is it worth it or should he just overhaul his 10hp
2. What problems should he anticipate associated with the smaller than spec prop. For instance will there be excess heat in the engine or perhaps carbon build up due to inefficient running.
3 If he did upgrade to 20hp with the 13" prop combinatin will the bearings and shaft be strong enough for the increased thrust and torque.

Phew!! that took me ages to type. I need to go and have a lie down and get my husband to make me a cup of tea!!!!!!

 

[mtb]

Is it a two bladed prop or three
cos he may be able to go for a three bladed prop which should keep the engine busy.The people who supply the prop should have the suitable list .The other way is to get a prop with a different pitch again ask the supplier.
The prop shaft size will be one inch ( or I'd be surprised ) and will certainly take up to 40 hp no problem .

Mind you Why run it at max with the added power there will be less noise cos the engine will not be flat out.
Cheers
Mick

http://homepage.ntlworld.com/boats
I want a steel ex trawler or tug cheap needing work

 

[Plum]

You can compensate, to a small extent, for a lack of diameter by increasing the pitch. However, talk to prop manufacturer/supplier and they will do the calculations for free. I did this with success on a previous boat. You can also compensate for a smaller diameter than is required for an engine by choosing a different gearbox ratio, but again, talk to the engine supplier/prop supplier and use their figures, then if it is not right they will often swap a new prop at no cost. Absolute minimun blade tip clearance is usually 10 pecent of diameter.