Engine power output

[Frank mellin]

My cruiser is a 24f t freeman design wit h a Nanni 20 ,hp. Diesel engine which is. Very reliable but seems to be lacking power.
With drive disengaged it will happily rev to 4500 r evs but when drive engaged will not go above 3500
I know that when driving the engine is under load but with or against tide makes no difference.max speed 6 knots
My question..can I check the engine output to see if in need of attention

 

[LittleSister]

Current Nanni 20hp - model N3.21 - lists 3,600 as max revs.

Most likely culprits if max revs can be reached in neutral but not in gear are
- fouled hull and/or prop (makes a big difference, increasing the load on the engine); or
- blocked exhaust elbow (reducing the power output of the engine).
A range of other potential issues available, but start with the above.

I am not familiar with the Freeman (or motorboats generally), so can't judge whether 6 knots is normal with 20hp.

The tidal current itself makes no difference to the engine load, you will go the same speed through the water either with it or against it. The movement of the water will increase or decrease your speed over the ground, but not through the water. (Tide can only have an indirect effect on speed through the water, e.g by creating bigger/steeper waves in wind against tide situations, or by creating overfalls, etc.)

Key things affecting load are
- the boat length and design (which is fixed);
- whether and how much the hull and/or prop is fouled;
- how heavily the boat is loaded and how well balanced (both of which affect engine load);
- speed (load increases with speed, increasing hugely as you approach 'hull speed'); and
- wind and waves.

 

[Dockhead]

Good comments above.

Just to emphasize, as LSister wrote -- be sure you are comparing like for like, i.e., speed through the water, NOT indeed SOG. And similar wind and sea condition.

There are conditions which can reduce engine output, injection pump fault, bad injectors, poor compression, but these are the least likely. And most likely noticeable -- either rough running, or hard starting. If the engine starts well, the compression should be OK.

You're not overpropped if the same prop allowed full revs in the past.

That leaves fouling. That's what I would look at.

 

[PCUK]

Did it rev to 4500 previously? Or is it as LS says, max revs are 3500?

 

[neil_s]

My 28 foot yacht has a 20 hp Bukh. It can just about get to 6.5 knots in calm conditions. Given that I have a longer waterline, I recon your motor is doing OK!

 

[LittleSister]

. . . Nanni 20 ,hp. . . .
With drive disengaged it will happily rev to 4500 r evs but when drive engaged will not go above 3500 . . .

Current Nanni 20hp - model N3.21 - lists 3,600 as max revs.

Both Nanni and Beta use Kubota base engines in this range. The Beta 20 appears to be the same engine as the Nanni N21.3 - i.e. Kubota 719cc 3 cylinder - and is specified as max 3,600 rpm.

I believe the fuel governor should limit the engine to the specified max revs, even in neutral. If it does not, and once you've checked the correct specified revs, you should avoid over-revving it in neutral.

Note that rev counters can misread if faulty. (You can check engine speed with a cheap strobe light sold for the purpose, or a phone app that uses the photo flash as strobe.)

 

[Plum]

My cruiser is a 24f t freeman design wit h a Nanni 20 ,hp. Diesel engine which is. Very reliable but seems to be lacking power.
With drive disengaged it will happily rev to 4500 r evs but when drive engaged will not go above 3500
I know that when driving the engine is under load but with or against tide makes no difference.max speed 6 knots
My question..can I check the engine output to see if in need of attention

Are you sure a Freeman 24 is capable of more that 6 knots with a 20HP engine? To me, 6 knots sounds correct.

 

[LittleSister]

A Google search suggests 6 knots is likely about right with a 20hp engine. A now deleted ad for a Freeman 24 (engine not specified) says cruising speed 4 knots, top speed 7 knots. A blog article on them says 9 to 10 knots could be achieved with a 50 or 55hp engine, or 14 knots with two such engines.

 

[Tranona]

Sounds like there is nothing wrong with the engine. The theoretical hull speed of a Freeman 24 is just over 6 knots in displacement mode. A 20hp engine is suitable for canal and river use, and at sea in calm weather. It has obviously been re-engined as originally it would have had either a Watermota petrol of around 30hp or diesel Perkins 4/108 or BMC around 35hp which would have given a bit more speed, probably 6.5 knots and feel more powerful (but increase your dental bills because of the vibration!). Maybe on reflection a 30hp Nanni or Beta would have been better for coastal use.

 

[LittleSister]

. . . originally it would have had either a Watermota petrol of around 30hp or diesel Perkins 4/108 or BMC around 35hp which would have given a bit more speed, probably 6.5 knots and feel more powerful (but increase your dental bills because of the vibration!).

And ramped up your fuel bills, too.

 

[thinwater]

The difference with more HP is in wind and waves. Tide, the water is moving, so no difference. And there is the whole hull speed thing. My 34' cat was about 0.6 knots faster with two engines than one. My 24' trimaran was about 0.2 knots faster with 6 hp than with the current 4 hp. The difference is when you are powering into strong headwinds. With more power you can maintain headway. With less power you slow way down.

Two ways the OP might evaluate power, since speed is little help, are:

• Bollard pull at the dock. You have to correlate based on props and some other things, but it will give an idea. My understanding is that 25-30 pounds per hp is about right. Reverse will be 30-50% less, depending on the prop. About 500 pounds, so nothing a piling can't handle. You'll need a cheap gauge.
• Powering into 25 knot winds with a reasonable seastate. If the boat slows to 4 knots or less some hp would help. If it slows 1-1.5 knots it's about right. A 24' boat is not going to make much speed into more wind than that without taking a serious licking.

Totally off the topic, but interesting to know about your boat. When anchored, measure the rode tension at about 20 knots (tension increases with V^2). Then measure bollard pull in reverse back at the dock. How do they compare? When you test or power set an anchor in reverse, how much wind is that equivalent to? With most boats, about 35 knots. With my anemic 4hp on a tri, and exhaust through the prop (outboard) it's more like 12 knots, but I only anchor the tri for fishing or swim/beach stops.

 

[Frank mellin]

Thanks for all the excellent replies.
I guess the answer was there all the time with a 3500 rpm max for the engine.
Appreciate that being brought to my attention.and all the other comments about the freeman 24 hull etc
Hull is clean just antifouled
Prop is free running
Guess ive just got to stop thinking im james bond and get used to sedate cruising.!
Again, really appreciate all the comments
Thank you

 

[PeterWright]

What's the waterline length of your Freeman 24? 6kts is the hull speed of a boat with waterline length of 18.4 ft. and for a conventionally designed boat, applying more power once you've reached hull speed just digs the stern in deeper until you climb up your own bow wave and start planing which is likely to need more like 100 hp. 7 knots is the hull speed for a boat of 25 foot waterline length, so a boat of 24 foot LOA is unlikely to reach that speed without planing.

You also need to check the accuracy of both your tachometer and speed log if you are worrying about a missing half a knot.

Peter.