Dear Forum friends, I have a Perkins 4108 with a 2.57/1 gearbox turning a 18x14 3 blades prop on a 52ft sailboat/42ft waterline. I would like to repower and I am interested in your ideas. Going thru the posts,I saw one from Stanley suggesting the Beta 43. My concern is about space in the engine compartment,minimal changes in silentblocks positions and position/height related to the shaft. The Perkins seems underpowered for such a large boat but,the prop is matched to 3000RPM max and we reach 8knots(no wind,flat sea)at 3000RPM. We cruise at 2400 doing +/- 6.5Knots. Cannot increase prop diameter(18')Short of clearance space. Perkins has a M65 that has been designed to fit in the same space than the 4108 and is a slow running engine but I would prefer a Beta. All ideas and suggestions welcome.Thanks in advance. Exbury II
Dear Stanley, Thanks for the answer. My gearbox is a ZF /HURTH HBW150 with a ratio of:2.63 foreward and 1.95 reverse. My boat weights 16 metric tons loaded. Since the 40/50 and 60 are the same size,I would rather go with the 60 even if it may not improve the performance a lot.If I decide to sell the boat one day,it may looks better to the next owner to have a 60 pushing that (relatively)big boat than a 40/50. Could a 60 be delivered with the same silentblocks spacing as my 4108?And would that resume as the same height for connecting the shaft to the gearbox? This would really ease the new installation. Thanking you in advance. EXbury II
If you go with the Beta 50 with 2:1 transmission the prop called for is 18"x13" three blade.
The Beta 60 calls for a 20" propeller. Can you accomodate the extra diameter and still have about 15% of the prop diameter as tip clearance?
My prop calc is based on 50% blade area ratio.
You must expect to drill new holes for the flex mounts. The mounts that perkins used had their bolt holes at 4" centers (100mm) whereas the mounts that we use are larger at 5.5" (140mm) spacing.
Dear Stanley, Thank you for the answer. I cannot put a 20' prop with a proper clearance.Only a 18' would do. I could use a 4blades feathering prop like the Variprop of 18' diameter. With a 2.6 to 1 ratio,I still have cavitation when the boat punch into a small wave.(with the 18X14 3 blades prop) Will it not be worse with the 2 to 1 ratio you suggest? No problem for the engine silentblogs.What about the height of the shaft to connect to the gearbox compare to the 4108?Is there a difference that would need to increase or decrease the height of the engine bed? Thank you in advance. Exbury II
Your Perkins is a much higher revving engine that the Beta Marine. Using your 2.63:1 transmission behind a 4.108 the max prop speed is potentially 1520 RPM. Using the 2:1 transmission behind the Beta 50, the max prop RPM is 1,400. If you are considering a Variprop, I suggest that you speak to them as they know how their props perform better than anyone else.
Today the normal arrangement on this size engine is to have the shaft at close to the same height as the engine beds. When your Perkins was new, there were a number of different mount arrangements available and you have to let us know what you have before I can comment on capability. If you look at this page of our website, you will find a drawing to complete which will help in determining the mount arrangement that you need http://www.betamarinenc.com/Special%20feet%20details.pdf
Dear Stanley, Thank you for your advice and your patience. After checking,the 2to1 ratio on the Beta 50 would give me +/-35HP for a shaft turning speed of 900RPM at 1800 engine RPM while at the same shaft RPM I have to have the Perkins 4108 at 2400RPM and have (theorically)+/-30/33HP At the maximum RPM recommanded by Perkins of 3000RPM the shaft turns at 1140RPM and should use(theorically)37HP. The Beta 50 at a shaft RPM of 1140 turn at 2280 engine RPM and should use 40+HP Now,I had the idea of a 4 blades feathering Variprop to increase the propeller surface and improve the cavitation that I notice when pushing against a small sea. Would you have any ideas about what cause this and how it could be improved? Thanking you in advance. Exbury II
Cavitation can be caused by different things. Without knowing your boat and how much of a fair lead it is for the water to the prop, it is impossible to give you any accurate answer. Sometimes the noise heard is prop wash against the hull which is annoying but doesnt really cause a loss of performance. It can also be caused by some propellers. As I dont know what boat you have and how her lines look, I really cant say much more. Stan
The propeller is right in the open behind a P bracket and has a very good water flow around it.
When the boat is slowed down by pushing thru a small wave,it sounds like you would be driving your car on a gravel road.
I suspect that my prop at about 45/50% ratio of diameter is lacking surface to grip properly and this is producing cavitation near the tip of the blades. But,I may be wrong and your ideas about this will be very welcomed.
I will measure the distance of my Perkins silentblocks towards the middle of the crankshaft and send them to you later.Any other measurements would be helpfull? Would you be helped if I did send you a picture of the mounts?
Your description of noise sounds like prop wash. It may be exacerbated bu a worn cutless bearing.
The measurements needed are 1) the distance between the mount centers port to stbd 2) the height differance between the engine beds and the prop shaft center. These are the basics needed to determine fit. Clear photos are always helpful but these are often difficult to get whith all the cabinetry in the way.
Sorry for the delay in answering but I was tied up in work projects.
Here are the measurements I took onboard lately.
The propshaft center is 36 MM(Millimeters) below the lowest point of the engine support. The engine support is where the silentblock is attached to the engine.It is not the bottom of the silentblock.
The distance measured at the front of the engine,between the shaft of the silentblocks is 455 MM(Millimeters).
Have you done a set of engine supports that would fit those measurements already?
Since the boat is now out of the water,I can attest that there is no play in the cutlass bearing.The edges of the propeller have lost the paint but not the rest of the blades nor the hub.I still suspect cavitation and wonder what could I do about it.
Please go to this page on our website and fill out the drawing with the measurements, http://www.betamarinenc.com/Special%20feet%20details.pdf Once we have the measurements filled in, we can see what mounts will work best. Regards, Stanley
Dear Forum friends,
