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Bertram31.com General Bulletin Board
Re: prop questions
Posted By: Gary Gibson In Response To: Re: prop questions (thuddd)
Date: Wednesday, 17 August 2005, at 5:49 p.m.
Tim,
The comments others have made about “square props”, that is three blade props with closely matching diameter and pitch values, is generally true with traditional diesel engines (which your 6v53s are) in planning hull applications utilizing traditional propeller blade designs (4 blades is similar but lower ratio due the fact that blade design generally provides ~20% greater blade are for a given diameter). As Paul indicated, this is a rule of thumb. Blade loading is another important factor. If not considered, you may have both excessive slip and blade flexing (which also changes the effective pitch). Your Hatteras 41 is a lot heavier than a B31 – and the loading of the prop (overall) will be much larger; requiring a larger effective area prop. You may want to consider Nibral props if you end up buying new ones as they flex less than bronze.While your boat was originally delivered with 24x23 props, she may be a bit heavier now and be better off with a slightly lower pitch. This situation is actually not an uncommon situation, even more so with new boats sold with high top speeds --- until you get all your gear on board.
If you have a pair of 24x21 (3 blade), I would put these on the boat (or modify to 24x22) and speed trial the boat FULLY LOADED. I would leave out cupping on the first trial (reduces cavitation losses if you are over the edge) With that speed information from these props, a good prop shop will be able to tell you what you need to do for optimum performance. The three or four blade issue really depends, under the right conditions the four blade will provide a better cruse speed (offset by lower top speed). Four blades may be a better choice due to blade loading…often four blades provide worse performance overall. If your local prop shops have not been much help, you may want to talk directly with the prop manufacturers (e.g., Michigan Prop.). I still recommend finding other Hatt 41 owners with 6v53s to find out what they have done and performance achieved.
I also suggest finding the original build data or at least the model number before re-setting the max unloaded speed. Cappy is right for the standard marine engine (standard governor, N50, standard blower gear, …) that you want 2800 RPM fully loaded (full fuel, water, gear and crew) – which means setting the Max unloaded RPM to 2900 – 2950 (assuming 150 RPM max droop for your particular governors). That said, there are engines out there which are set up for 2700 RPM (or other) loaded . The m55 injectors you mention (I think they are now labeled N55), if original, may have had different factory settings than models with the more common N50 injectors. These injectors have also been added to many engines increase HP. If so, was the gear ratio for the blower changed (this may limit the max RPM) ?
Gary
P.S. the book Paul mentioned “The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers” by David Gerr. provides excellent background information on the subject.
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