Bristol 35.5 prop selection

[dwfosterjr]

I own hull number 1 (Robin) and it was repowered with a Yanmar 3GM30F and it feels underpropped. I have not hauled the boat to get the existing prop info but it is a 3 blade. Hull displaces 15,000 pounds. RPM for hull speed is 3500 which is way high for my comfort regardless of the torque curves and high RPM design. I would like more thrust in forward and reverse. I have heard that the maxprop requires aperature modifications and that the Luke may fit-- anyone with any experience of diameter and pitch etc would be great--thanks--wf

[westwinds]

According to the engine manual the Yanmar 3GM30F is rated at 27 HP at a maximum of 3600 RPM. For continuous operation the maximum recommended HP is 24 HP at 3200 RPM for a variable prop. For a fixed pitch, run the engine at 3400 to get 24 hp for continuous operation. The point here is that if you reduce engine RPM, you reduce HP. If you believe the boat is underpowered, then you will need a larger engine as the 3500 RPM is going to get almost the maximum HP. You could go with slightly less pitch to get to 3600 and 27 HP. The Luke pitch is adjusted manually with an alan wrench so if you drop your RPM by increasing pitch, you will not have maximum HP if needed. The Max Prop is also adjusted manually, same problem about max HP if needed. If the noise is an issue, just drop your RPM to a noise level you are comfortable with. You will also save considerable fuel. If you run at 3500 RPM you will use about 4 times the fuel than if you run at 2000 RPM but of course the boat will be slower; however, you will get about 2.4 times further on a gallon on a calm day. This is for a fixed pitch prop. A propeller that varies the pitch with the RPM will get even better economy.

[malbert73]

Originally Posted by westwinds

According to the engine manual the Yanmar 3GM30F is rated at 27 HP at a maximum of 3600 RPM. For continuous operation the maximum recommended HP is 24 HP at 3200 RPM for a variable prop. For a fixed pitch, run the engine at 3400 to get 24 hp for continuous operation. The point here is that if you reduce engine RPM, you reduce HP. If you believe the boat is underpowered, then you will need a larger engine as the 3500 RPM is going to get almost the maximum HP. You could go with slightly less pitch to get to 3600 and 27 HP. The Luke pitch is adjusted manually with an alan wrench so if you drop your RPM by increasing pitch, you will not have maximum HP if needed. The Max Prop is also adjusted manually, same problem about max HP if needed. If the noise is an issue, just drop your RPM to a noise level you are comfortable with. You will also save considerable fuel. If you run at 3500 RPM you will use about 4 times the fuel than if you run at 2000 RPM. This is for a fixed pitch prop. A propeller that varies the pitch with the RPM will get even better economy. Memory says about 30% better, but do not quote me, I could be wrong on that.

I agree with the above in principle, but doesn't quite fit the OPs question. He only achieves hull speed at max RPM. He should be able to achieve hull speed in flat water in that boat with less RPM, with plenty HP at higher RPMs in reserve for rough conditions. If on the other hand, the OP can't pitch up and stillget max RPM, then that would mean not enough HP. My Luders 33 which displaces around 13500 can get 6.2 knots at 2800 RPM with a 14x14 michigan MP3 prop on a Yanmar 3YM30 and still can achieve max RPM with that prop. That indicates to me that whatever HP I get at 2800 RPM is what is needed for flat water near hull speed. Most of time I run at 2600 RPM and get 6.0 knots and save a ton of fuel.

[westwinds]

I did come prop calculations on an Alberg 30 about a year ago for a fellow. At 2000 RPM the boat made 3.3 knots, at 3400 5.6 knots. At 3400 RPM the HP was 16 and at 2000 RPM, 3 HP. The gallons/hour was .24 at 2000 RPM and 1.2 at 3600 RPM. The miles per gallon was 13.57 at 2000 RPM and 5.75 3400 RPM, at 3600 RPM the miles per gallon was 5.75. The take away here was that 2000 RPM gets you more than 2.4 further than 3600 for a little more than half the speed at 2000.

[Curmudgeon]

The first calculation you have to do is whether or not the prop you select with fit inside the aperture. It helps to make cardboard template of the blades.

[westwinds]

Also consider tip clearance. It should be at least 15% of the diameter from the tip of the propeller blades to the bottom of the boat. This is for propellers that turn at 1800 RPM or less. If you have a gearbox with a 2:1 reduction, then engine RPM can be 3600. Since the bottom of the boat slopes upward at the stern, the shortest distance may not be straight up so move that ruler back a forth a bit. The tip clearance is necessary to prevent vibration and improved efficiency from reduced turbulence. One other thing, the diameter of the propeller shaft should be 1/14 the diameter of the propeller or more. This would be the value for stainless 304; however, Aquamet and Monel are considerably stronger (a smaller diameter can be used) and are also resistant to crevice corrosion. Stainless crevice corrosion occurs because an inactive yacht will have places where oxygen cannot reach the stainless propeller shaft and the protective coat of oxidation will be lost. In this case, stainless has the corrosion resistance of mild steel in those spots. This is also why galvanized bolts are better than stainless for keel bolts. Silicon bronze is a better material for keel bolts than galvanized.

[dwfosterjr]

all good info and many thanks. I will measure aperture when the boat is hauled. As I understand it a larger prop, even though it may fit in the aperture and not exceed 14 times the shaft diameter (1 inch + 14inch), the pitch of a three bladed prop may load up the engine and prevent it from reaching sufficient HP to drive the boat any faster or with more forward or reverse thrust in sea's or while docking. If the existing prop lets the engine reach and not exceed max RPM the issue may be an engine lacking the HP for better perfromance. As the boat cannot stop in 3 boat lengths at 3 knots I was hoping for some additional power for stopping and driving through a headsea. The latter of which reduces speed to about 3.5-4 knots in a moderate 3-4 foot sea (not chop).

[dwfosterjr]

I have a good Autostream 15 X 12 (adjustable pitch) feathering prop I want to use but don't know if it is too much for the Yanmar 3GM30F with a 2.62:1 gear ration. Anyone with a 15,000 poung sailboat have this engine and any advice?

[MarkSF]

My Bristol 31.1 with the Universal 24hp does 6 kts at 2000 rpm, and won't rev past 2500. Clearly it's a bit overpropped based on max rpm. But boat performance is excellent. I was motoring into chop last weekend, and it only slowed down to 5.5.

This is by way of saying that a coarser prop might not extract maximum hp from the engine, but it might result in better performance for you.

I heard that Bristol offered a 2 blade, and 3 blade prop as options.

The 35.5 is only listed with the Universal 24hp - so it sounds like someone did an engine swap to a Yanmar on yours? Strange choice, I'd have put in a Beta and re-used the engine mounts.

Sounds like they also swapped the prop because you wouldn't be doing 3500 rpm with the original 3 blade.

Suggestion : find out what it is, go one step coarser. At this stage I suspect your prop aperture will accomodate a considerably larger OD and/or pitch than you have, because I suspect what you have is pretty small, and that aperture was sized for a big, coarse, prop.