Replacing a perfectly good Yanmar 2GM20

U

ump010

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www.yachtelectricalservices.co.uk
You probably think I’m mad, but I’m replacing a perfectly good Yanmar 2GM20 with A Beta 25, so as to have a bit more power.

Now my boat has a long keel with the rudder on the aft end of the keel. There is an aperture for the propeller. I obviously need to upgrade the prop to handle the extra power. I can’t increase the diameter of the prop, aperture not big enough, so I have to increase the pitch.
I am told I can have the prop re-pitched, save buying a new one.

Does anyone have any experience of re-pitching props? Does it work?

I must get it right first time, because to change the prop I would have to haul out, move engine in to the saloon and then draw prop shaft in to the boat, (rudder only about 25mm aft of the prop nut).

So, re-pitch or buy a new prop? Opinions welcome, thanks in advance!
 
How big is your boat? And how heavy?

At first glance it seems a lot of trouble and expense for what may turn out to be a hardly noticeable increase in power.
 
How big is your boat? And how heavy?

At first glance it seems a lot of trouble and expense for what may turn out to be a hardly noticeable increase in power.
 
I'd co,nsider taking some pretty close measurements first for the size of the proposed new prop.


If you have little clearance already between the deadwood and the rudder, there might be even less with a bigger pitch - especially on full rudder lock. This will interfere with the prop's efficiency and might produce a knocking sound as the blades pass near the aperture edges.
 
Boat is 30ft, about 6 tons.

Already have the new engine coming and old one out.

Have plenty of room forward of prop (about 6 inches to deadwood) and aft to rudder about 3 inches, just not enough room to slide the prop off the shaft! No problem with rudder hard over.

Thanks for replies.
 
You need to go back to first principles and size your prop to suit your new engine and reduction gear ratio, Suggest you feed your data into the Propcalc programme on www.castlemarine.co.uk which will give you a good guide to whether you can prop it correctly (should have done this before you bought the engine). Then go to a prop manufacturer/supplier with the same data and ask them to recommend a prop. It may be that you can just coarsen the pitch on your existing prop, which a specialist can do. However, be aware that it is often difficult to get a prop big enough to absorb the extra power when you are limited in diameter - probably why your boat has a 20hp engine in the first place. You probably won't see much improvement with your extra power, simply because you can't get a prop to use it.
 
Yes. I had mine done by www.tnorrismarine.co.uk. They can adjust the pitch by up to 2 inches. My boat was over-propped for "economy", or so the previous owner reckoned. I had the pitch reduced by 2 inches and was delighted with the result and with the service Norris provide - the propellor was returned looking like new with the new dimensions stamped on the hub. Has performed faultlessly in the seven years since.
 
Red Admiral said:
Yes. I had mine done by www.tnorrismarine.co.uk. They can adjust the pitch by up to 2 inches. My boat was over-propped for "economy", or so the previous owner reckoned. I had the pitch reduced by 2 inches and was delighted with the result and with the service Norris provide - the propellor was returned looking like new with the new dimensions stamped on the hub. Has performed faultlessly in the seven years since.
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Sorry to hijack the thread, but can you tell me the approximate costs of the repitching by Norris' and how long it takes to get the job done?
 
Most props intended for a sailing boat have quite a narrow blade so as to minimise drag when sailing. Likewise, it is common practice to use a two bladed prop and some people will mark the shaft so as to be able to lock the shaft with the prop aligned vertically.

If you have problems with getting the prop to transmit the available power then you can go for a broader blade type such as is used on a motorboat or a threeblader or both. Using too great a dameter prop for the aperture will disrupt the flow across the blade, reducing its efficiency and quite likely causing vibration.

Rob.
 
Paul at First Marine Propulsion is a A1 chap who knows props inside out. They are based in Shamrock Quay, Southampton. www.1stmarine.com

I would and have recommended him (them?) to one and all.

Simes
 
A properly-pitched prop with a 20HP engine should be more than adequate to push a 30-foot 6-ton boat at its maximum displacement hull speed. (we're not talking about a planing hull here, where you can get more speed with a bigger motor). I know you've already spent the money, but it seems to me like a lot of effort and expense for very little, if any, reward.
 
Repitching

mickywillis said:
Sorry to hijack the thread, but can you tell me the approximate costs of the repitching by Norris' and how long it takes to get the job done?
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I had our prop repitched by T Norriss last season. Service was excellent and for a 17" prop, it was about £110 and was turned around in about 10 days. It came back lovely and shiny and has increased speed at cruising revs by about a knot and a half. Worth doing if your prop is in good nick. In fact the service was so good that my dad bought his new folding prop from them this year too.

Rob
 
greenalien said:
A properly-pitched prop with a 20HP engine should be more than adequate to push a 30-foot 6-ton boat at its maximum displacement hull speed. (we're not talking about a planing hull here, where you can get more speed with a bigger motor). I know you've already spent the money, but it seems to me like a lot of effort and expense for very little, if any, reward.
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If I recall correctly, the GM20 is only rated at 16HP continuous output.
I changed mine for a perkins perama engine, (about 24HP continuous) in a
26' bilge keeler of about 4 tons- it transformed the boat, quieter, less vibration, a slight increase in top speed but at cruising revs far superior.
It doesn't stall when cold if the batteries are down, and the boat now has the power to punch through a chop. And I will have a calorifier fitted next year!
 
I don't know much about this so this might be a daft question, but doesn't all this depend on the speed these two engines run at? If the Beta delivers its higher power at proportionally higher speed wouldn't that require the same prop?
 
CreakyDecks said:
I don't know much about this so this might be a daft question, but doesn't all this depend on the speed these two engines run at? If the Beta delivers its higher power at proportionally higher speed wouldn't that require the same prop?
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Rev range of the two engines is very similar, so assuming the reduction ratio is the same (ie same shaft speed) it will be able to swing a bigger prop - either diameter if room or coarser pitch. The difference in actual shaft hp is about 5 - 16 for the Yanmar and 21 for the Beta.
 
I've just replaced my underpowered 2GM20 with a 3YM20 for the same reason.
I already have a 3 bladed feathering Kiwi prop with adjustable pitch.
You could consider the same!
 
Always nice to have a new engine but I would not expect too much-I have just gone the other way and replaced "a perfectly good 30 Hp engine " for a new 20Hp engine. Most of the time very little noticeable difference in power and boat speed. I suppose when the time comes I need to push hard against wind and tide under motor I will miss the extra HP but so far it has been all good.
 
The 2GM is a wonderful little engine. Better than the later model 2GM20 in my opinion. You should still get the Yanmar service manual.
 
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