Swan 41/Optimum pitch for a Perkins ...
...Prima 50hp, Borg 2.1:1 and Max Prop 3B 15

I have read with a lot of interest the previous very informative threads on propeller pitch, pleasure speed and commercial speed, gear box reduction ratios etc… and was wondering whether the professor or a fellow S&S Association member would eventually hold some of the necessary information to determine the optimum pitch on a Swan 41 with a Perkins Prima 50hp, a Borg-Warner 2.1:1 and a Max Prop 3 blades 15" to avoid the trial and error process (classic with pitch adjustment).
Would anyone have for example the propeller curves for a Perkins Prima 50 (which I guess are different from the 4.108)?
Many thanks in advance!
Matt

...Prima 50hp, Borg 2.1:1 and Max Prop 3B 15 I have read with a lot of interest the previous very informative threads on propeller pitch, pleasure speed and commercial speed, gear box reduction ratios etc… and was wondering whether the professor or a fellow S&S Association member would eventually hold some of the necessary information to determine the optimum pitch on a Swan 41 with a Perkins Prima 50hp, a Borg-Warner 2.1:1 and a Max Prop 3 blades 15" to avoid the trial and error process (classic with pitch adjustment). Would anyone have for example the propeller curves for a Perkins Prima 50 (which I guess are different from the 4.108)? Many thanks in advance! Matt

Apologies for the typo… . (Most of you will have probably noticed). The Max Prop fitted on the boat by the previous owner is a 3 blades 17" and not a 15" (too small for the 41) as written inadvertently.
Rest is as mentioned.
Thank you!
M

Dear Matt
The recommended pitch setting with the given parameters is 22 degrees for 3000 rpm maximum.
Prima M50 is identical to Volvo MD22L, and Volvo allows this engine to be braked to 2600 rpm. If you would like to use the same approach the pitch should be set to 24 degrees. You will then reduce the maximum power to about 45 hp, and use slightly lower cruising rpms for the same speed.
One inch is a rather coarse unit in the calculations, and I would appreciate if you check whether the diameter is exactly 17".
Kind regards
Lars

Dear Professor,
Thank you very much for the prompt and precise answer!
I had taken references from the Volvo MD22 L already thinking it would/should not be far.
I have to take the boat out in the coming days and will measure the propeller precisely and get back to you in case the size would differ and not be the 17".
Thank you, once again,
Matt

Dear Professor,
The boat was taken out of the water, size of the propeller checked (it is indeed 17") and pitch set at 22 deg as advised. Seems indeed optimal for the configuration of engine and gear box.
As previous engine set up had a smaller exhaust creating back pressure (and certain overheat) it is indeed difficult to make a proper comparison - all things being equal - (I have indeed changed the size of the exhaust pipe, including he size of the water-lock in the meantime and the engine clearly performs "breathes" better), but for sure further sea trial will determine whether some more fine-tuning is needed for top and cruise speed with this pitch.
In anyway, thank you again for your speedy answer!
Subsidiary question if I may: what is your view on the hull speed formulas?
The limitation to the waterline without any factoring of the displacement or hull shape sounded rather like a (educated) approximation.
As I did not have any hull speed for the 41 (I am sure you know) i came with LwL formula at a value of 7,37 knots.
Can you kindly share your view (if not already done in a previous thread)?
Many thanks in advance
Warm regards
Matt

Dear Matt
Thank you for the information.
The basic check is to verify maximum rpm at full throttle, and the corresponding speed. With 22 degrees it should be close to 3000 rpm, and your hull speed calculation is correct, this is what you can expect.
Finding out if there is too much back pressure would require provisions for measurement on the exhaust elbow. If too high it can be reduced by branching off part of the cooling water, so less is injected into the exhaust, but to determine exactly how much is a demanding task.
The maximum recommended cruising rpm at 75% output is 2700, this should give 7.2 knots, but it is assumed you prefer a lower output with less fuel consumption. Pls note that minimum recommended cruising rpm is 1900, this corresponds to 25% engine output and close to 6 knots.
Kind regards
Lars

Thank you Professor!