How to tell whether I'm over or under or just right propped?

Aeolus

Aeolus

Well-Known Member
Joined
3 Aug 2004
Messages
1,026
Location
Sussex
Visit site
As per the heading, how can I tell whether I'm over-propped, under-propped or whether the propellor is just right?

The engine is a Beta 20 and I usually cruise at 2500 rpm which achieved ~6 knots in smooth water just after launching (so with a clean hull). The boat is a Seamaster 925 sloop, LWL 7 metres, long fin, displaces approx 4 tons.
 
That sounds right for cruising revs and speed. Test is whether you can achieve close to maximum rated rpm which is 3600 and about 6.8 knots. 3400 would be a minimum.

If you want to check the size of your prop put your boat data into the Propcalc programme on www.castlemarine.co.uk to get a suggested size.
 
When I had my then boat's MD11C replaced by a Nanni 21 the installer had to measure max. Revs. In neutral and in gear as part of the commissioning procedure. As far as I recall the figures were 3600 and 3400 rpm. Which were deemed OK.
 
Aeolus said:
Hi Minchsailor,



I can see that sizing the prop will depend on the gear box ratio but why would testing whether I'm over or under propped depend on the ratio?
Click to expand...

You are right - it does not make any difference as it is fixed. So the prop is either the right size or not. Just as a guide 1" of pitch will make around 250-300 rpm difference with a 2:1 reduction which I guess you have.
 
Aeolus said:
As per the heading, how can I tell whether I'm over-propped, under-propped or whether the propellor is just right?

The engine is a Beta 20 and I usually cruise at 2500 rpm which achieved ~6 knots in smooth water just after launching (so with a clean hull). The boat is a Seamaster 925 sloop, LWL 7 metres, long fin, displaces approx 4 tons.
Click to expand...

Why has nobody mentioned hull form? Position of the prop, is it a saildrive or shaft drive? The engine power and speed is important but means nothing if the other stuff is not taken in to consideration. See if you can find someone to run a standard programme to calculate the prop characteristics based on the below water profile. They are available and with the right info you can obtain the prop that most closely matches your requirements.
 
One good indicator of overpropped is when the engine won't achieve max power rpm with the boat securely tied to the quay.
That can be too much pitch or too much diameter/blade area.
It's underpropped if the boat speed is too low at max power rpm.
 
lw395 said:
......
It's underpropped if the boat speed is too low at max power rpm.
Click to expand...

Raises the question of what speed is too slow w.r.t. max continuous power rpm when a margin of power is required to counter head winds/seas. I am in the process of specifying a new prop and want to cater for progress in adverse conditions at the expense of higher fuel consumption or reduced cruising speed. Recent experience of bashing to windward on an overpropped boat was worrying in respect of low speed and engine loading.
 
Last edited:
There's no easy way of telling if you are underpropped - that would mean that that the engine never develops maximum power but that is hard to detect. If you have all the numbers you can type them into a prop calculator and see what it says - should give a good indication
 
AHoy2 said:
Raises the question of what speed is too slow w.r.t. max continuous power rpm when a margin of power is required to counter head winds/seas. I am in the process of specifying a new prop and want to cater for progress in adverse conditions at the expense of higher fuel consumption or reduced cruising speed. Recent experience of bashing to windward on an overpropped boat was worrying in respect of low speed and engine loading.
Click to expand...

If the engine has adequate power for the displacement of your boat and you have chosen a reduction ratio to allow you to swing the largest diameter prop you can fit, then a correctly pitched prop will allow you (as in the OP's case) to achieve cruising speed at around 70% of max power and hull speed at close to rated maximum rpm. For many modern engines this equates to cruising around 2200-2500 and max 3600, although some engines are governed to lower maximum rpms giving correspondingly lower cruising rpms.

As an example, my last boat had a Volvo 2030 which cruised at 2400 and maximum 3500 (out of 3600) whereas my new boat has a D1 30 which is rated at max 3200, so cruises at 2200 and max achieved is 3100.

So you will have the additional power to cope with adverse conditions, but your ability to do this will depend firstly on the efficiency of the prop - a 3 blade for example would normally be better than a 2, and the boat characteristics such as windage and hull shape.
 
lw395 said:
One good indicator of overpropped is when the engine won't achieve max power rpm with the boat securely tied to the quay.
.
Click to expand...

Are you sure? I think not. The load on the engine is so much that it won't reach max rpm. I know this from first hand. My boat used to reach maximum rpm of 3500 (with previous gear box and propeller; now I am overprop) but tied to the quay only around 3100.
 
lw395 said:
One good indicator of overpropped is when the engine won't achieve max power rpm with the boat securely tied to the quay.
Click to expand...

I'm with cmedsailor in disputing this, since it requires less force to spin a prop when it's the boat it's attached to it moving. It also depends on the type of quay and how the vessel is orientated to it. A boat stern-to a solid wall will absolutely not develop anything like it's open-water revs.
 
macd said:
I'm with cmedsailor in disputing this, since it requires less force to spin a prop when it's the boat it's attached to it moving. It also depends on the type of quay and how the vessel is orientated to it. A boat stern-to a solid wall will absolutely not develop anything like it's open-water revs.
Click to expand...

Stern to might be taking it a bit too far, but if you find yourself aground and can't get the engine to rev, you will soon come around to believing your boat is over-propped.
Likewise if you are trying to smash through waves that stop the boat.
Or you could just think it's underpowered.

It can be a compromise.
Underpropping can be more of a judgement about extracting the last fraction in flat water calm.
 
My engine, Volvo MD11C in a Fisher 25 revs up to 2800rpm when tied to the pontoon and in forward gear - and produces lots of black smoke at this rpm

However when underway it wont go above approx. 2200rpm - at this rpm the boat is doing approx. 5.8kts speed over ground, which is hull speed (clean bottom, prop)

My conclusion is therefore that the engine can never reach full rpm when under way as it is limited by the hull shape/weight of boat?
 
cmedsailor said:
Are you sure? I think not. The load on the engine is so much that it won't reach max rpm. I know this from first hand. My boat used to reach maximum rpm of 3500 (with previous gear box and propeller; now I am overprop) but tied to the quay only around 3100.
Click to expand...

I said 'max power rpm', not max rpm.
3100 may be about right?
Bearing in mind, it is the real max power that matters, for your engine on the day, not theoretical figure for a brand new engine.
If you'd said it would only do 2500 on a bollard pull, I'd be suggesting it was over prop'd.
 
gavin400 said:
My engine, Volvo MD11C in a Fisher 25 revs up to 2800rpm when tied to the pontoon and in forward gear - and produces lots of black smoke at this rpm

However when underway it wont go above approx. 2200rpm - at this rpm the boat is doing approx. 5.8kts speed over ground, which is hull speed (clean bottom, prop)

My conclusion is therefore that the engine can never reach full rpm when under way as it is limited by the hull shape/weight of boat?
Click to expand...

It's over propped and the prop stalls on a bollard pull.
 
You must log in or register to reply here.

Share this page

Top