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Old 08-30-2016, 11:16 AM   #7
Al
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City: ketchikan, Alaska
Vessel Name: 'SLO'~BELLE
Vessel Model: 1978 Marben-27' Flybridge Trawler(extended to 30 feet) Pilothouse Pocket Cruiser[
Join Date: Apr 2012
Posts: 2,206
Don- In general boats of similar yet different design perform differently.

Our hull is termed a SD, although given the performance, one would be inclined as we are, that it is more a displacement hull demonstrated by performance.

By calculation our boat shows a suggested prop somewhere around 19X13 3 blade. we are running a prop of 22X16. 3 blade.
Our specs used are 14000#, 25 LWL. 8.5 Beam at waterline, 2.5 draft ex keel depth (1foot) 85Hp at 3000 RPM.
Our current conditions as a result are a max of 2000 RPM WOT. Hull speed at WOT (Run at 5 min full WOT) was determined to be 8 knots. We run at a max of 1400 RPM resulting in an average of 6.5 knots. Our fuel burn as reported, averages 1.5 GPH.
As we maintain a reserve of 600 RPM or 1/4 of available RPM, the normal recommended for marine engine use as I understand the general understanding, we are content in the engine performance.

The Perkins 4-236 which is the model, has a wonderful reputation for being one tough platform for heavy duty application.
Given the manufacture gives three levels of RPM measurement, 2500 RPM for heavy application (My interpretation of the wording from memory) 2800 for intermediate duty, and 3000 for light duty.
My assumption is the 3000 with proper gear and prop is the more correct application intended for our boat.
To achieve that requires our changing to a 3:1 gear from current 2;1 or the recommended small replacement prop per the formula result.
The end result would increase our running RPM and increase the noise level with perhaps a anticipated fuel burn reduction to near 1 GPH,

So, in conclusion Don, we are way out of wack according to all the book calculations. yet,we have achieved near calculated hull speed (6.9-7 knots) at a reduced RPM,retaining a engine RPM margin,and with a satisfactory fuel burn with a quite confined pilot house noise level.

Hence the earlier comment of not going to defend what is working for this application.
other than to say, the end result was gained through 'seat of the pants' trial and error.

In reading djmarchand's response, there seems general agreement to much of our efforts and results.

Regards,
Al-Ketchikan, 27' Marben pocket CRUISER
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