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Old 27-08-2016, 14:20   #1
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Prop size Vs Horsepower???

I just bought a Tartan 34C and am thinking of changing the engine to a diesel. I understand that the T34C is limited to a 12 inch prop because of the strut location. The recommendations seem to be for 25 to 30 HP diesels, but I wonder just how much HP a 12 inch prop can actually absorb. The direct drive Atomic 4 really only has about 16 to 18 HP and I wonder if a much larger engine is going to make much difference, if you cant put a larger prop on? How high of a pitch can you go before you are just churning water and would a 12 inch Max Prop or other adjustable prop be OK with 25 or 30 HP? I dont want to waste HP, weight or fuel for the sake of saying I have a big engine. Advice requested. _____Grant.
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Old 27-08-2016, 15:33   #2
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Re: Prop size Vs Horsepower???

Once you max out blade size the next step is to add more blades, so switch from a 3 to a 4. Then you can play with the pitch as well. Worst case you will be under proped and have reserve power you just can't reasonably use. The upside is if you wind up motoring into a major wind you will still have excess power.
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Old 27-08-2016, 16:08   #3
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Re: Prop size Vs Horsepower???

You are right that the direct drive Atomic 4 can't produce anything close to the rated 30 HP. Another issue is the direct drive which results in high propeller RPM which means an inefficient propeller. I suspect a 15 HP diesel will power the boat as well as the direct drive A4. The limitation on propeller diameter is a problem. As has been noted, going to a prop with more blades will give you more blade area which is a good thing. As to pitch, it's driven by the propeller RPM. The lower the propeller RPM (i.e. higher transmission gear ratio) the higher the required propeller pitch.
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Old 27-08-2016, 16:57   #4
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Re: Prop size Vs Horsepower???

Right now it has a 3 blade, but it also came with the original 2 blade that looks like a watch charm. I am thinking that the difference between a 15/18 HP diesel and a 30/35 HP would leave me enough money to buy a 3 blade Max prop and play with the pitch. I really dont want a fixed 3 bladed parking brake on the boat. Time will tell. This is a project boat so 3 times the cost estimate and 3 or 4 times the work time. That is probably how it will work out. Thanks guys, Grant.
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Old 28-08-2016, 06:00   #5
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Re: Prop size Vs Horsepower???

I assume you've looked at the Tartan 34c sites which have info on diesel repowers. Looks like most go with 20 or maybe 25 HP to give them some margin for powering in rougher seas and to run a higher output alternator. I think I would go for a 20 HP and a folding or feathering prop would be a great plus over a fixed 3-blade. FWIW my Cal 33-2 is about the same size and displacement as the T34c, and the Yanmar 3GM30F with a folding 2-blade prop easily drives the boat to over 6 knots. Of course I have space for a 16 inch diameter prop.
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Old 28-08-2016, 12:36   #6
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Re: Prop size Vs Horsepower???

The Campbell Sailor Prop is smaller in diameter than a normal prop for usually better thrust. I just bought a Campbell for a 2qm15 yanmar, dropped two inches in diameter.
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Old 28-08-2016, 14:44   #7
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Re: Prop size Vs Horsepower???

My P35 prop diameter is limited by the aperture to about 13". Boat was converted to a Yanmar 3GM30f from an A4. Transmission for the Yanmar was the lower numerical geared 2.21 ratio. Prop was 13x12 and seemed just about perfect for the boat/engine/gear ratio.

In a fit of Ebay madness bought a Sailor 13x13 prop. Engine would not go beyond 2800 rpm with lots of smoke with that gearing and the Sailor Prop.. Switched to the higher numerical geared 2.6 transmission at the recommendation of the Sailor people and all is now well with the engine able to run up to the 3600 rpm redline. Listen to the Sailor people if you decide to go with their prop. Using the conventional prop size recommendations for the sailor prop just doesn't work. Anyone want a very low hour Kanzaki 2.21 geared transmission???

Think you are still a sailor so doubt you'd be happy going to a multi blade prop. The prop is very exposed on those boats and any prop drag will be felt in sailing performance.

The Tartan 34C was my first choice for a boat when I began looking to get back into boat ownership. Unfortunately, they were scarce as hen's teeth on the Left Coast and the one or two that were for sale were priced higher than what I though they were worth. I'd be happy with a 20hp engine in that boat and think the optional diesel was just a little over 20hp Fairyman. I'm not the kind that likes to pull skiers so don't favor big engines. The 27hp Yanmar pushes my heavier Pearson just fine and have never had it anywhere near redline, except to test it, in real life use. As you know, was very happy with the 20+ hp Volvo with a two bladed prop in our 20,000# W32. There seem to be a few smaller diesel take outs from the water skiing types who want a larger engine so might find a good used engine in the 20hp range. Didn't you buy a used 2 cylinder Volvo in anticipation of getting back into sailing?? Height is an issue with the engine in the T34C because of it's midship location which may prove an issue in what engine you go for.

If the boat has a wheel and you want a tiller, make sure they didn't cut off the rudder shaft extension where the tiller attached after they converted to a wheel. Most of these boats were tiller steered and only later converted to wheel by idiots.

If you haven't found it yet, there used to be a very active T34C owner's group website. They were very helpful with any questions in my long distance search for a T34C.
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Old 28-08-2016, 17:20   #8
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Re: Prop size Vs Horsepower???

work out optimum rpm range for your prop/rpm of choice of diesel/choose reduction or step up box to mate them together at your chosen preference/get out here and sail so as not to wear out your costly stink power
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Old 28-08-2016, 17:34   #9
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Re: Prop size Vs Horsepower???

Call these guys, or send them a question on their website:

https://www.generalpropeller.com/ind...te=common/home

They're the best prop shop I've ever dealt with and will have the answer to anything prop-related you can ask them.
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Old 28-08-2016, 18:00   #10
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Re: Prop size Vs Horsepower???

Roverhi, I think 20 HP will be plenty, but the fit is an issue. I dont want to raise the seat/enginebox height if I can avoid it. Yes, I had a 2 cylinder Volvo that I bought for a boat deal that fell thru. It and an Aries and a boom gallows and a set of mast steps all burned up in last years Valley Fire. My sexton and lots of other gear burned. I had been gathering gear to go out again. I had it all in a shipping container. When they would allow us back into the property the container looked like it might be OK. It had actually turned into an oven and burned so hot it melted the injection pump off of the engine. That set me back for a time, but I am moving forward. Thanks everybody for the info. _____Grant.
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Old 29-08-2016, 01:00   #11
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Re: Prop size Vs Horsepower???

Im sorry for loss in the fire. Glad you are back into it.
Repowering is certainly a big undertaking, even if youve done it before.
General propeller are good to work with. I did have a problem with shaft /coupling tolerances with them.
I know its not for everyone. But i suggest check out Electroprop .
You will have batteries in your boat right? Eliminate oil and fuel issues, stink and filthy bilge, tankage, thru hulls and more. Clean and neat.
Just a suggestion.
Have a good one.
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Old 29-08-2016, 07:39   #12
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Re: Prop size Vs Horsepower???

Crunching the numbers for the T34 I find the following:
Displ. 11200 lbs
LWL - 25'
Beam max 10'
Theoretical hull speed- 6.5 kts.

I used a made up engine spec of 3600 RPM max and a 2.2:1 reduction ratio, and plugged that info into my prop program, getting the following results:

- 15 h.p.@ 3600 RPM---Max speed 5.7 kts-- prop- 13.5 X 8 X 3 blade

- 20 h.p.@ 3600 RPM---Max speed 6.1 kts--prop- 14 X 9 X 3 blade

- 25 h.p.@ 3600 RPM---Max speed 6.4 kts.--prop- 14.5 X 9.5 X 3 blade

The prop diameters listed are the smallest diameters that the program will allow for the given h.p., without driving the prop into cavitation at higher loads and RPMs. Going to a 4 blade prop doesnt change the equation significantly, and probably isnt worth it in terms of cost and drag under sail.

Given that you have an existing diameter restriction of 12 ", your best bet is probably to limit your power to about 20 h.p. and install a prop with as much blade area ratio as possible. That could be something like a 14 or 15 inch prop with the diameter cut down to 12 inch.

Something to think about..

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Old 30-08-2016, 08:01   #13
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Re: Prop size Vs Horsepower???

I pulled the boat out of the water yesterday and the 3 blade on it is 10 inch diameter. It looks like such a toy. On the T34C forum there was a suggestion to go with a longer prop shaft and a strut to get the prop a little further out and be able to be larger diameter. It will probably depend on what engine I end up with. I am not a believer in big HP for cruising, so I may stay with the 12 inch limit and go with a Max or some other feathering 3 blade, and a relatively small engine. Time and budget will dictate what happens. Thanks for all of the info. _____Grant.
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Old 31-08-2016, 08:22   #14
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Re: Prop size Vs Horsepower???

Grant, the T34 has the same waterline length, very similar beam, and quite close in displacement to the Catalina 30, which also had a similar diameter limitation. It might be worthwhile checking C30 sites, or asking on the very active C30 Yahoo group.
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