Hunter 30 biggest engine?

[twobitdad]

I am looking to buy a 1978 Hunter 30 that is in great condition. Either the current or a previous owner has upper the engine from the Original Yanmar to a 24 hp Bukh. I was told that he also changed the prop to a three bladed brass... My concern with upping engines is with the shaft strut and shaft thru hulls. Does anyone know or have an opinion on this?
It has been in place for over five years and has over 100 hours on it without an issue...

Thanks,

 

[Barnacle Bill]

First of all props are not made out of brass, but out of bronze. What is your concern exactly? If the repowered inboard was properly installed, then I should see no problem.
A shaft through hull? You mean the shaft log? Stuffing box? Not sure what your concern is????

 

[twobitdad]

There are brass props but that is not the concern or question, it was just to point out the propulsion system has been changed from the original design.

By through hull I am referring to the hull parts, not the interchangable parts that are inserted into that opening, i.e log, shaft seal, etc.

The engineers/designers made the engine mounts, transom, hull structure, around the shaft, the shaft strut, etc. to handle the forces needed to push the boat at a given speed using a given toque which was determined to be the selected engine and prop (blades and pitch) delivered with the vessel. Any changes to one of these would require the considerations for the designed strenghts of the others. For example, the Hunters, as with most boats, will experience prop walk, which is the sideways motion caused by the transfer of energy laterally to the shaft due to the bite of the prop blades and resistance of the boat to move in the line of the shaft. When a boat prop walks there is a higher than normal lateral force applied to the shaft, shaft strut and shaft log as well as the any shaft unions or linkage. The engineers take this into consideration during design.

So my concern is, by doubling the available HP of the engine there is also an increase in the forces that can be applied to the propulsion components, does this stay within the designed strength characteristics of the boat or is the this a risky situation that could cause structural failure of the hull or propulsion components?

 

[KD3PC]

there are props made of some material containing small amounts of brass.. ie bronze contains some small amount of brass, nibral as well and so on...but I would say that there are NO brass props except models or decorative....

and it should be a concern...because if you have a real brass prop that will cause some issues, and should be addressed sooner than later.

but to your question of added horsepower...no problem with it, as long as your motor mounts, bearings and shafts, etc are in good shape. Those forces you describe are of minor affect as compared to the "lower" hp stock engine

bigger worry is for spare parts for that Bukh engine...

 

[twobitdad]

Thanks for the info, I can verify the mechanical elements during the survey.

 

[Benny17441]

If the drive shaft is round and in balance there should not be a problem. Theorically a stronger engine can drive a larger or heavierly pitched prop but all that energy should be absorbed by the shaft itself. It would be the adequacy of the shaft that would be in question an not the structure around the hull. Let's anylize some of this, the maximum HP output for an engine are attained at near Max RPM. At lower RPM it is likely the larger engine will not be placing any more strain than the smaller engine at a faster RPM. Also consider that when backing up or turning any strain placed on the hull attachment and cutlass bearing are really a function of the water resistance to allow boat motion. As the water resistance is the same with either engine it could be said that there will not be any significant additional pressure on the joints. I grant the larger engine can provide additional acceleration which could increase the stress in some conditions but here is where a measured throtle activity by the operator can negate any additional stress. These hull structures are overengineered to allow for stress under adverse conditions and some accidents. We have all wrapped a line on our prop shaft at one time or another with no apparent ill effects. Like twobitdad mentined I would be more concerned about finding spare parts for the engine.

 

[twobitdad]

Benny, great info, thanks....i was thinking of the max HP and not the actual or applied...15 HP is the same no matter what size engine applies it....
What you stated is what is was expecting to hear about the energy transfer but i don't know the design basis of the boat hulls so had doubts, thanks for the confirmation.