How to find the gear reduction on a Borg Warner
Hi all,

I would like to optimize our Max prop propellor pitch.
For the moment we are doing 4 knots at 2100 rpm with our Swan 44. I think this is a little bit to slow.

The engine is a Perkins 4.108 with a Borg Warner transmission. The serialnumber plate is lost.

I took some pictures and one is showing a interesting number: could it be 2.88 gear ratio?

I don't know what horse power was used on the 44 but on the 43s with 37 horse power Perkins the Borg Warner reduction transmission was 1.91:1.

Hiro

We gave a Perkins 4.108 with 55hp, the one with dry cilinders (the latest model). The older one with 37HP has wet cilinders.

Serial number engine: ED70058U 041845T

Maybe the old Perkins and the newer models have a different gear reduction?

Perkins engine number explaned: ED70058 U 041846T

ED = Engine family
70058 = Parts
U = Build in the UK
041846 = serial number
T = 1989 (Year of manufacturing)

So our Perkins 4.108 have 55HP, maybe with a different gear reduction.

Dear Ice and Hiro
According to information from Perkins both the 43 and the 44 used the same type Velvet Drive AS3-70CR with reduction 2.10:1.
Hiro suggests 1.91, but according to Perkins this reduction ratio was not used on any of the engines supplied to Swans. In case Hiro has hard evidence this would confirm that old information is not always reliable.
Would suggest that Ice uses a portable rpm counter to read the shaft rpm, and also checks the reading of the rpm counter on the engine panel.
When the reduction ratio has been determined the propeller diameter (accuracy 1/8 inch pls) and number of blades are required in order to give a pitch recommendation.
Referring to the comment that the engine is a 55 hp version, pls note that it is not a good idea here to use the high speed pleasure rating with 4000 rpm max for a sailing yacht. The propeller then needs to have low pitch producing low speeds unless high rpms are used.
The recommendation is to use the commercial rating with 3000 rpm max, and set the pitch accordingly. At 2100 rpm you should then expect to cruise at 6 knots
Kind regards
Lars

Many thnx for the input Professor.

Dear ICE,

I face the same problem and have a friend coming to my boat this weekend to check the rpms at the shaft and the engine and panel.  I hope the panel and engine match!

Good luck!

Chris

Dear Professor,

Would mind explaining the reason for matching a prop on a sailboat that has the Perkins 4.108 to meet 3000 rpms rather than the user manual recommended 4000 rpms?  You may have already explained this to me, and possibly others, and I am sure it's somewhat complicated, but I am very interested to learn. 

Thank you.

Chris Mabel's Casse Tete 43/003

Hello everyone:

All I can say is that my Borg Warner transmission has a tag that indicates a 1.91:1 reduction gearing. I know that my engine was rebuilt once under previous ownership and they may have switched the transmission at that time. The perkins engine serial number is 108U75098.

As of last year I have been swinging a Gori folding 3 blade propeller 16.5" x 12 x 3 blade (LH). I previously had a Martec folding 2 blade 18" x 16 x 2 blade (LH).


Max rpm for both propeller is just above 3,000 with 2,500-2,700 rpms used most instances for a 5.5-6 knots of boat speed.

Hope this helps.

Regards,

Hiro

Chris
I will try to explain this with the help of the propeller curves.
Engines are sold based on their power curve, but in marine applications the propeller curve determines the power going out into the water, and this is the dominating curve, also denoted the propeller law. Engine leaflets usually show both curves.
For Perkins 4.108 there were three ratings, we will deal with two of them here – High Speed Pleasure, and Commercial. The ratings are 47 shp/4000 rpm respective 37 shp/3000 rpm, and old engine leaflet curves are appended here below. It is preferable to compare shp (shaft horse power) and not some higher fictive numbers used for marketing. An international standard ISO 8665 for the declaration of engine power has been created in order to compare apples with apples for pleasure boats, and it is advisable to check whether the engine manufacturer refers to this standard.
It has been tempting to sell the 4.108 engine with the High Speed rating. If engine dealer A offered you a Perkins 37 hp, dealer B would sell you a 47 hp version, and dealer C claims he has a 51 hp engine, would you expect to pay the same? The difference between the versions is in the setting of the high idling rpm, is done with a few turns of a screw on the injection pump. You can easily find out which version you have by checking the high idling rpm – full throttle in neutral gear.
The Commercial diagram shows an interesting detail, Point 4 Minimum allowable rated speed. Commercial engines are expected to work hard, and this is the minimum allowed full throttle rpm for example when towing, or in a strong headwind. It is important here that the high idling rpm is set correctly, otherwise the injection pump tries to achieve 4000 rpm. If you have the Commercial setting and the corresponding propeller curve you could try what rpm is achieved at full throttle with the yacht held stationary, should be close to 2500.
Engines are also often compared on a hp/weight basis. Low rpm engines are preferable for sailing yachts, but can be expected to have a less favourable hp/weight ratio.
So much for the introduction, now to the comparison.
If we assume that 12 propeller hp is required for a cruising speed of 6 knots, the propeller curves show that with the High Speed rating close to 2500 rpm is needed for this power, with the Commercial rating about 2000 rpm. It is easier to compare the two if both propeller curves are shown in the same diagram, pls see the third diagram below. A propeller sized for the Commercial rating uses 82% of the High Speed rpm for the same power, and at the same rpm absorbs 76% more power. This is valid over the whole rpm range, and can not be affected with the reduction ratio.
The High Speed version can produce more propeller power than the Commercial only in a narrow range when the rpm exceed 3700. A common misconception is that the High Speed version produces more power over the whole rpm range, but the propeller curves show that this is not the case.
It would be a different matter with an engine version producing 47 shp/3000 rpm. When comparing engines it is therefore advisable to compare their propeller curves and not merely the power.
The engine manufacturer determines the point of maximum power through which the propeller curve should go, and the shape of the curve is roughly a 3rd degree parabola. Perkins used the exponent 2.8 here, for example Volvo uses 3.0. This information refers to ordinary fixed pitch propellers. Sizing the propeller so its curve goes through the desired point can be done by trial and error, or by calculation. It would be much easier to choose propellers if their curves would be specified instead of their dimensions.
The shape of the propeller curve can be affected by using a controllable pitch propeller, but this is a completely different matter, and not covered here.
Kind regards
Lars

Sorry, the third diagram was shrunk, new try
Lars

Dear Lars:

Thank you for this most detailed explanation.

Regards,

Hiro

Dear Lars,

Thank you so much for your thorough and clear explanation. I have read it slowly and will need to re-read it and then I think I may have a question or two.

With warm regards,
Chris

Hi Lars,

If i show Maxprop the 4.108 perkins high speed pleasure curve above together with the transmission reduction they can calculated the perfect pitch? Or do i need more numbers?

Regards,
Ice / Swan 44 "Repo Man"

Dear Professor,
Thank you again. I have re-read your explanation. My engine, in neutral, registers over 4000 rpms. Do I need to adjust the high pressure pump (something I do not want to do) or can I leave the engine as is and, after we determine the reverse gear ratio, determine the appropriate propeller to run at max 3000 rpms when under power?

With warm regards,

Chris

Dear Ice
The curve is not needed, it is sufficient to state the hp and rpm for the chosen rating, and probably the maximum propeller diameter.
I have a feeling that Max-Prop will suggest that you "tune" your propeller, guess this means increasing the pitch to reduce the rpm. My advice is that the engine needs to achieve 3000 rpm at full throttle.
Kind regards
Lars

Chris
You can leave the injection pump as is if you avoid using full throttle. The engine will probably smoke if you use it.
Kind regards
Lars

Dear Professor,
I am afraid of working on the high pressure pump, probably out of ignorance compounded by scattered information from various articles, but I do have a friend who is an exceptional mechanic. I plan to pass him your recent essay on the Perkins 4.108 and why we should set the max rpms to 3000. I think he will find it enlightening. Then, maybe with the incentive of a beer or dinner, I can get him to work on the injection pump per your suggestion!
Thank you again.
As always, I am in your debt. Without you and this forum (thank you Matteo), I believe I would have given up on this boat long ago.
Chris

I also agree with Chris.

This web site and forum is a pretty amazing place, especially with our access to the Professor and his vast knowledge of not only the construction details of the Swans but his engineering expertise.

Invaluable information and help.

Hiro

Chris
For your information the high idling rpm to be set to 3380 rpm for the 4.108 with 3000 rpm rating.
Kind regards
Lars

Dear Lars,
Thank you. I may have mentioned that the yard, very small, had to launch me to make room to launch other boats, so, while in the water, I hope to test the gear ratio. Once we determine this, I will be, once again, call on you for help in determining the proper prop!

I will also adjust the injection pump as you suggest - with trepidation!

With warm regards,

Chris