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post #32 of Old 08-24-2013
Roger Long
Retired Naval Architect
Join Date: May 2012
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Re: what kind of headwind should a sailboat be able to motor through?

Is that 2300 RPM with the throttle right up against the stops? Check the linkage to be sure the lever in the cockpit is moving the lever on the engine the full distance.

If so, that is your problem but other things could compound it.

With the boat clean and light, as your was, the RPM should come right up to maximum continuous in smooth water and calm winds. You should check with a manual or Yanmar for that RPM but, if it is an older Yanmar, 2600 sounds right. More than 50 RPM less than this, and you are hurting the engine and it will falter under heavy load as you are experiencing.

That is a an important limit on proper pitch. You can also have the prop WOT match at the higher rating most yacht engines list as being for "one hour of operation only". This will let you move slower through crowded mooring areas, maneuver a bit better, and have slightly more ability to punch into head seas and wind. It's like being in a lower gear in your car and most people prefer it. The drawback is higher fuel consumption while cruising. Since I cruise full time, much of it under power, I prefer the higher pitch. The engine run quieter at lower RPM and the fuel savings add up.

If you are over propped, send the prop out to a shop, tell them the WOT RPM you are getting and what you would like to get. They can bend the blades while re-balancing and reconditioning and get very close to what you want. It costs less than $200 here in Maine for a prop of that size.

For the severity of what you are experiencing though, you must also check for fuel and air restriction. If it is that heavy black smoke you see trucks make pulling away from a stop, it probably isn't fuel restriction. Just be sure you aren't seeing smoke only because you look anxiously over the stern when the engine falters. All diesels smoke some and it looks heavier as you slow down because the boat isn't moving away from it as fast.

If the engine is simply faltering and slowing, it could well be fuel. The silly little tank vents they put on yachts plug with corrosion or wasp nests. Enough air can get in to run at low cruise RPM and wind load but the fuel pump can't pull against the vacuum enough to get more fuel. Old fuel lines can also have separation of inner layers and other blockages. Make sure your tank doesn't have a screen over the suction inside the tank that is getting blocked with scum. This is the time to get that tank well cleaned out. If that tank has sludge on the bottom and biofilm on the sides, that new filter you put in could have been partially plugged up again within the first hour you ran the boat. You can't just dump something from a bottle in the tank. That will just make it worse. You need to talk to a boatyard or do some research. It's a whole subject of its own. If you have inspection ports, there is nothing like rags and elbow grease.

Do you have an electric fuel pump? My tank gravity feeds and runs fine at cruise RPM but the engine begins to falter exactly as you describe (except for the smoke) if I go to full power. I've given up trying to remember to go down and turn on the electric pump and, since full power is usually something you need when a lot is going on, I just leave the pump on all the time.

Air restriction is often overlooked if your engine is in a tight space. Diesel's need lots of air. Too tight an engine box or an oil soaked foam filter element (which I think your engine has) can cause the symptoms you describe. Remove the air cleaner and leave a hatch open and run hard. If it runs properly, that's your problem

Last edited by Roger Long; 08-24-2013 at 07:05 AM.
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