Transmission on Swan 41

For three years are the owner of " Cial II " 41 S&S/04 , I write because I ask my advice on adjusting the propeller of the boat , which are not very convinced . Do not ask my propeller speed , but I would like to rotate the motor at a higher speed . The data on which reasoning is : The boat I own for three years, is a Swan 41 S & S in 1974. The LOA is 12.50 m, width of 3.64 m, the waterline length is 9.22 m, the draft is 2.10 m . The gross tonnage of 11.91 tons , displacement is 10.8 tons , the lead bulb has a weight of 4.4 tons . The previous owner in 1994 replaced the engine fitting a Yanmar 4JH (2) E -aspirated direct-injection by the power of 50 hp , which currently has 1,300 hours of sailing. I read in the manual, which is mounted an trasmission KMA147 R 1.47 / 1 . In transmission shafting , propeller with three blades max prop diameter of 440 mm . (Measured by me) , with clockwise rotation , replaced by the previous owner in 2000. With adjustment of the propeller at 18 ° , the motor boat is 7 knots quite easily , at least in maneuvering clicks but does not exceed 2,100 rpm of the engine, accelerating after smoke and endeavors . I would not want to increase the speed, but I would like the engine reached 3000 rpm , also to have a reserve of power . I did decrease the step , I gained 300 rpm motor. The boat is currently dry , and I would like to lighten the propeller pitch, taking them from 16 ° to 14 ° , according to you is a wise thing ? I think that if the pitch of the propeller can also lead to the 12th or 10th there will be a reason.
Thanks in advance .   ggalizia Cial II/04

Dear Ggalizia
Thank you for the information.
The 4JH2E engine has a maximum rpm of 3600, and the engine manufacturer requires that this rpm is reached at full throttle, otherwise there is no warranty given. The problem here is that the chosen reduction ratio 1.47:1 is not suitable, and produces too high propeller rpms and limits the maximum rpm. The propeller needs to have very low pitch, but this makes it inefficient, and at rpms over 2000 the high rotational speed causes reduced propeller performance, and there is very little reserve power. This reduction ratio is intended for planing powerboats.
All this means that you now can use about 17 hp of the engine power efficiently at 2000 rpm, higher rpms produce rapidly increasing losses while quickly overloading the engine.
Reducing the pitch to 14 degrees will increase the maximum rpm another 300 rpm, but this goes into the pitch regime which generally has to be avoided because of very low efficiency.

If you want to use more engine power it is necessary to change the reduction ratio, the recommendation is 2.63:1 which is mentioned as an alternative for this reduction gear. It should be noted that this requires the pitch to be set to 22 degrees, and the use of 2600 rpm in order to cruise with 17 hp as before. With this pitch setting the engine should achieve 3600 rpm maximum, and provide ample reserve power.
Kind regards
Lars

Thank you Lars, for the answer, you were clear.
Do you still want to ask you what would you do in my place?
Would you leave everything like that, or would you change the gearbox?

Greetings ggalizia S & S 41/04

Dear Ggalizia
With the present set up the engine power has a rather limited usable range. On one hand the risk of overheating the engine is real - you should watch the cooling water temperature closely whenever you use more throttle. On the other hand you should not use too low rpms when cruising under power, the engine will then collect a lot of soot, and this finally leads to serious problems.
With the present 16 deg pitch setting the minimum recommended rpm is 1700, if you reduce the pitch this rpm is 300 rpm higher.
If the engine still is in good shape - you should also measure the compression - the advice would be to change the gear ratio, but if there are engine problems this is not worthwhile.
Kind regards
Lars

Dear Ggalizia,

I faced the exact same problem you are currently facing; with the help of the professor, I decided to swap out both the transmission and propeller. I am doing the work myself which means very slowly! However, having made the decision to do this, I am happy that I did. I know it seems like a lot of work and, if you're like me, you may wonder how the reduction ration for a displacement boat could be so low...in my case, I wonder but have moved on.

Good luck with your decision.

Chris Mabel's Casse Tete