Slow speed and propellor size

[kevsbox]

Hi
Our yacht is a 8m Macwester Rowan Rose (long keel), with a displacement of about 6,270 lb.
She is fitted with a Yanmar 1 gm 10 which is serviced yearly and the propellor is 3 blades 14.05 by 10.5
I do not have a tacho fitted but with the lever halfway round we seem to get the best speed but this is at max 4kn and when fitting against the tide coming into harbour it is often 1kn
While the engine is not the most powerful I would have hope for a bit better performance and I am wondering if the propellor is the best size (it was replaced shortly before we purchased the yacht)
Any thoughts please

 

[Graham376]

Sounds like you may be over propped. What happens if you increase revs past half, does it blow black smoke?

 

[Tranona]

First and most important thing is to check the reduction ratio. The 1 GM comes with a choice of 3 and the one you should have for a 14" prop is the 3.1:1 option. The pitch should be 9"rather than 10.5 and the difference is losing you 350-400rpm. However, although your theoretical hull speed is 6.14 knots you need 16hp to get near that. The best you can hope is for 5 knots in flat water and taking 1.5" off the pitch so you can access an extra nearly 2 hp will get you close to that.

I had a very similar boat (Eventide 26) in size and displacement and did a lot of work extracting maximum performance out of the weedy 1GM. The fixed prop with a 3.1:1 was indeed 14*9 and could exceed 5 knots and cruise at 4.5 knots at just under 3000rpm. Later I changed to a feathering prop that was specially made for the boat. 15" with a pitch of about 7.5 which was a little faster and of course better under sail. Even later I replaced the engine with a Nanni 14 (a 16 was too long to fit) with a 2.65:1 and the same props. This was much better achieving close to 6 knots on test, although I sold the boat with the fixed prop so don't know how it performed in real use.

For various reasons the new owner did not take the feathering prop so I still have it. It has been fully serviced by Darglow (who made it 20 odd years ago). If you have a 1" shaft (which is almost certain) you are welcome to try it as it is otherwise sitting in my study looking nice and shiny. I have not tried to sell it because it is so specific, but could work well on your boat. I am in Poole. Send me a pm if you are interested and I will take some photos for you.

 

[kevsbox]

Sounds like you may be over propped. What happens if you increase revs past half, does it blow black smoke?

I do get some black smoke

 

[Tranona]

Usual symptom of overpropping. The governor is pumping in more fuel to meet the demand you are placing on it with the lever and the fuel is unburned. Can eventually do damage to the engine. Short term, just bring the lever back to the point at which the black smoke stops.

 

[kevsbox]

First and most important thing is to check the reduction ratio. The 1 GM comes with a choice of 3 and the one you should have for a 14" prop is the 3.1:1 option. The pitch should be 9"rather than 10.5 and the difference is losing you 350-400rpm. However, although your theoretical hull speed is 6.14 knots you need 16hp to get near that. The best you can hope is for 5 knots in flat water and taking 1.5" off the pitch so you can access an extra nearly 2 hp will get you close to that.

Good point on the ratio, forgot about that. I assume this is on the gearbox plate. I will be on the boat Wednesday so I will have a look. Sadly that thing called work gets in the wait of playing about on the yacht.

 

[Tranona]

Yes, it will be on the plate. Pretty sure it will be that ratio as you would not even get it to move more than a couple of knots with the smaller reductions which usually have a 12 or13" prop!

 

[bristoljim]

Good point on the ratio, forgot about that. I assume this is on the gearbox plate. I will be on the boat Wednesday so I will have a look. Sadly that thing called work gets in the wait of playing about on the yacht.

Very important to know what RPM you get at wide open throttle - can you get hold of a hand held tachometer ?
Jim

 

[kevsbox]

So I checked the gearbox plate today and the ratio would appear to be 3.22. Also checked the shaft is 1". So is the best prop size to be a 14by9?

 

[kevsbox]

Very important to know what RPM you get at wide open throttle - can you get hold of a hand held tachometer ?
Jim

I am looking into that

 

[Tranona]

These are the calculations from Vicpropvicprop.com/displacement_size_new.php?action=calculate which we used when experimenting with different propellers. You will see that I have used 8hp as this is the continuous rating for the 1GM. As I said earlier a 14*9 worked very well achieving a bit more than predicted at 3600rpm and cruised at about 4.5 knots at 3000rpm for many hours - Poole to Cherbourg, Alderney to Poole - 14-15 hours more than once!

Your easiest solution if the prop is in good condition is to get it repitched to 9". The 15" feathering JF propeller I have will, based on the data that I gathered increase your maximum speed by between 0.3 and 0.5 knots (or reduce your cruising rpm for a given speed). Average passage times under sail improved by a similar amount - that is around 10%, significant on this size boat (1 hour shorter Poole Cherbourg) if you do longer passages. I am happy to lend you this prop to try with no obligation as otherwise it is just an ornament!

 

[kevsbox]

So we are now looking at removing the prop that the next suitable low tide. The rudder etc have all been removed.
I have been looking at prop pullers on ebay such as this one, Inboard Prop Puller Works on 3/4" to 1-1/8 shaft Replaces for # PULR-PT-POINT UK | eBay.
Today I stuck my hands under the water and had a good feel of the end of the shaft, this shaft was fitted about 2 years ago and the is no dimple on the end.

All the pullers I have seen refer to the dimple so can I still safely use this type or does anyone know of a similar device that will suit better?
Cheers
Kev

 

[andsarkit]

I made up a puller similar to this with a couple of bits of scrap steel plate and some studding.

It doesn't have to be round or look pretty.

 

[Poignard]

I made up a puller similar to this with a couple of bits of scrap steel plate and some studding.
View attachment 146449
It doesn't have to be round or look pretty.

I made one like that and it works perfectly and cost little.

 

[kevsbox]

Thanks chaps, having seen what they are to buy new I can see why you made your own

 

[bristoljim]

Thanks chaps, having seen what they are to buy new I can see why you made your own

Dont forget a decent propane torch is very useful in getting props off.
Jim

 

[billskip]

Thanks chaps, having seen what they are to buy new I can see why you made your own

Just make sure that you are not behind it when it let's go...

 

[ChromeDome]

I am looking into that

If you don't want/need to install a permanent tachometer, universal laser gizmo's can be found on all marketplaces.

Just fit a bit of reflective tape to any rotating object (flywheel), point the beam at it read the rpm from the display

A random example: