Engine Not Achieving Max RPM

[gavin400]

HI,

I have a new Beta 30 in a Fisher 25 with new 15x9, 3 blade prop

I can achieve the engines max rpm of 3600 in neutral so throttle cable/actuator is getting full travel
When tied to the pontoon and in gear I get max 2700
On the water I can only reach 3100 - at these revs the boat is doing over 7 kts
Is it the case that the boat is reaching hull speed and therefore the engine cant produce any more revs?

The boat has only been in the water for a week so its not got fouling
The prop size was recommended by Beta - I dont know if they did any calculations or just went with what they have previously recommended
There is no smoke at max revs

I am very happy with this performance, just curious to know why I cant get close to max stated rpm and if the current set up will damage the engine

Whilst I wait to hear back from Beta I would very much appreciate any thoughts

Thanks

Gavin

 

[Tranona]

Right on cue - I was testing my new prop settings this afternoon. And was happy with 3100/3200. If you do the sums you only need around 25hp to get comfortably over hull speed so no point in going up to 3600. However you don't want to "overprop" to much so it overloads the engine. 3100 is fine. You could take a inch off the pitch to get the revs up, but you won't get any more speed.

My set up is not directly comparable as I have a 2.5:1 reduction so that I can use a 17" prop. Originally I was going to have a 2.65:1 TMC box so the prop was pitched at 17*11 which turned out to be too much for the new box limiting revs to under 2900. changed the pitch last week (easy with a Featherstream) to 10" and 250 more rpm, same speed. Happy.

Think Beta will be OK with your speed and revs.

 

[gavin400]

Right on cue - I was testing my new prop settings this afternoon. And was happy with 3100/3200. If you do the sums you only need around 25hp to get comfortably over hull speed so no point in going up to 3600. However you don't want to "overprop" to much so it overloads the engine. 3100 is fine. You could take a inch off the pitch to get the revs up, but you won't get any more speed.

My set up is not directly comparable as I have a 2.5:1 reduction so that I can use a 17" prop. Originally I was going to have a 2.65:1 TMC box so the prop was pitched at 17*11 which turned out to be too much for the new box limiting revs to under 2900. changed the pitch last week (easy with a Featherstream) to 10" and 250 more rpm, same speed. Happy.

Think Beta will be OK with your speed and revs.

Tranona,

Thank you for your quick reply - engine revs and prop sizes seem like a bit of a black art when you are a novice

 

[geem]

We are over propped, same as you. Maximum revs is 2400rpm on the engine but we only make 2000rpm. No smoke and very good fuel economy. We can do 7kts in to big seas with a 22" prop and 8.5kts in flat water. At 6kts the revs are only 1200rpm in flat water with 3.3 litres per hour fuel economy for an 18t boat. Drop to 5kts and 1000rpm and 2.5l/hr. You may find you get good fuel economy for not so many revs

 

[oldmanofthehills]

I think we too are over propped as brand new prop was bought for a different engine which I rejected under warranty. Abortive engine had lower max rev so prop is coarser than ideal for final one

So our Beta 25 does not do its supposed max revs under load and only gets about 3100 not 3600. As boat can reach hull speed of 6.5kt well before that I am not over concerned. Seems very economical at about 1litre an hour for 5 ton boat at about 2000rpm so clearly not wasting much power, so do not wish to change back to original prop which would be underpropped

 

[jwilson]

Slight overpropping is possibly good as it means that at lower speeds or when sail-assisted the revs are lower and a bit quieter. If you can shove a heavy-ish tubby-ish Fisher 25 with a waterline length of 21 ft at 7+ knots I think you are getting plenty of power for the hull.

 

[Tranona]

Perhaps worth clarifying the impact of changes in pitch or diameter of props on revs. The rule of thumb is 1" change equals 250-300 change in rpm on the sort of engines we are talking about here. Shown clearly in my change. Dropped from 11" to 10" revs rose by approx 250 (difficult to be precise with closely spaced markings). Speed much the same but with the 11" pitch signs of overloading the engine, labouring at max rpm. at the other end load on engine was too high at tickover, causing problems with the transition from governor control of fuel to fuel rack. Hunting and boat going to fast. New pitch has cured that and trickles along nicely at tickover.

With the OPs (and my) boat the 30 is a bit overpowered - a 25 would just make hull speed, but would need to run at 3600rpm to do that. We have both chosen the larger engine - not so overpowered, no need to run at rated maximum and perhaps most importantly cruise at 300rpm lower. No effect on fuel consumption as power required is the same.

There is an element of black art in getting the optimum match of engine/reduction/propeller for individual boats, but now engines are available with enough power to exceed hull speed for most yachts, the "standard" recommendations that Beta give are a good compromise, With my GH we wanted to go outside the "norm" and use a 17" rather than 15" because the blanking effect of the long keel means getting the blades out and having bigger blade area would improve motoring performance. Of course this might not be desirable with a fixed prop, but with a feathering type drag is less of an issue. Using a Featherstream makes changing pitch easy. Did it last week in under an hour while in slings. You can also have different pitch forward and reverse, so I opted for a lower pitch to give more progressive control at lower revs - avoids that big "bang" when selecting reverse. The larger diameter needs a slower shaft speed, so a 2.5:1 reduction rather than the standard 2:1.

If it all sounds a bit anal, I learned a lot about how to extract the maximum thrust out of limited power at Seagull in the late 70s, developing gearboxes and propellers for the "new" range. Later did a lot of work with my Eventide which was grossly underpowered with a Yanmar1GM. That had a 15" feathering prop and a 3:1 reduction. Still had to run it at 3000rpm to cruise at 5 knots with maybe another half knot in reserve, but a big jump from the Stuart Turner!

 

[bignick]

Effective power is proportional to v^3, so a little extra speed takes a lot more power.
7 knots from a Fisher 25 is really good going and I wouldn’t expect you to be able to get any more than that.
if you can’t get the max rated rpm then typically you are torque limited, I.e. slightly over-pitched on your prop. That said, if you’ve got no black smoke then you should be ok And I think if I was able to get 7 knots from that boat then I’d be more than happy with the performance!