nelson 29 ford sabre 90 hp enging rpm issue

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Hi All

So i have a Nelson 29 with 2 ford sabre engines 90Hp but wont reach max rpm not even when she is fully cleaned and dry and just put in the water she is reving 300 rpm lower than her intended max rpm any ideas what can be done to solve this problem she has no black smoke under power or ideal

also while moving and she hits a wave or swell the engine rpm reduces till she is back on flat water

also if waves are from the back of the boat and she is riding the wave she over revs

your help is greatly appricated

Regadrs
Kurt
 

burgundyben

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I guess these are Dorset or Dover block?

What rpm do you get?

With gears in neutral to you get full revs?

How many hours have they done?

When you take off the oil filler cap, are the crankcase breathing much?
 

Bilgediver

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It would appear from you post that your engine may be overloaded. This is normally identified by the engine ceasing to react to increases in throttle movement and if this is the case the throttle should not be moved beyond this point .

You seem to relate that both engines are operating under similar conditions and reacting in similar ways and that the hull and props are clean . Is this a new boat to you or have you owned it for some time and are you comparing engine speed with known previous performance last season or with performance figures in a manual ? One other thing you could check when next on the boat is that on both engines when you move the throttle for an engine to full speed position the speed lever on the governors of both engines travel the full distance to the full speed stops on the fuel pump/governor and confirm this here.
The engines DO NOT need to be running for this test.

It would help if you could answer these questions and the questions from others about the actual RPM under different conditions.
 
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yes very
Sounds overpropped.
It would appear from you post that your engine may be overloaded. This is normally identified by the engine ceasing to react to increases in throttle movement and if this is the case the throttle should not be moved beyond this point .

You seem to relate that both engines are operating under similar conditions and reacting in similar ways and that the hull and props are clean . Is this a new boat to you or have you owned it for some time and are you comparing engine speed with known previous performance last season or with performance figures in a manual ? One other thing you could check when next on the boat is that on both engines when you move the throttle for an engine to full speed position the speed lever on the governors of both engines travel the full distance to the full speed stops on the fuel pump/governor and confirm this here.
The engines DO NOT need to be running for this test.

It would help if you could answer these questions and the questions from others about the actual RPM under different conditions.
I have checked that the leaver on governer moves all the way when on wot also i am comparing with manual figures as maxium speed shoud be 16kn at at 2500 rpm and i only get 2300 rpm and to 12kn
 

QBhoy

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Hi. Often a help to know that the stated engine rpm max range figure is usually not something that all boats with that engine will always be in accordance with. To be pitched to perfection on the prop for all influencing conditions that a boat can be subject to, is surely an impossible thing to see consistently. Impossible to even expect even. To be pitched in the props within 300 rpm isn’t too bad to be fair. As you’ll likely know...things like having a 100 litres less fuel or not having a crew member on board in addition, could easily see you gain some rpm. Jeez, I know with my own boat, just having the hood up, the other half and wee one on board and a few supplies and gear for a day out at the beach, will see me drop 300 rpm and a good few mph easily, should I open the throttle...compared to just me on my own with the hood off. Should you be bothered by it enough, it would be good to run your numbers through a prop slip calculator and see what slip she sees. But I’d suspect you could easily pick up or drop 300rpm very easily between any given day, conditions, tide and gear or passengers on board.
 

penfold

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The Nelson is a semi-displacement design, if the props were matched to the engines you shouldn't be getting a variation of hundreds of rpm unless you have a wedding party on board.
 
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