hi

[soloauditor]

Hi i am new to this site and i need advise on a project i am working on i have a heavy motor sailer 1972, 16 ton 2 mast which looks like a fishing trawler hull with a 6.7 litre volvo md 70c .38ftx12ft a very unique boat trouble is the propeller has been stolen and i can not find any info on the original prop size. i have used an on line prop calculator but the boat so heavy it is unlikely to sail without the engine running so it would be engine plus sails on a cruise.is the prop size and pitch different because of this? can anybody help i can not afford any fancy props as i am on a tight budget.

[GordMay]

Greetings and welcome aboard the CF, soloauditor.

[djmarchand]

Boatdiesel has a pretty accurate prop calculator. But we will need your transmission ratio to figure a recommended prop size. Post that and I will run it for you. BTW I assume 16 tons is metric tons, right? and is the fully loaded weight. I also assume that your waterline length is about 35 feet.

BTW a Volvo MD70C is a beast of an engine for that size boat. It should run forever. Also what speed and engine rpm did you cruise at (no wind) with your former prop?

Sails will take some of the load off of the prop. With that much power to weight ratio I would significantly overprop it, ie more diameter and pitch than would otherwise take to hit 2,500 rpm at wot.

David

[soloauditor]

Thanks for the help , waterline about 35 ft x 11 ft draft 4.5 ft. 2 to 1 transmission max rev 2500.@140 hp but at cruise of 1800 rev 100 hp
I bought this as a project in poor condition but i want to put it in the water soon you are right it is a beast of an engine for a sailboat i believe this was ex military so it never " existed " is built like a tank and has a tank engine and transmission.It is boat number 11 for me and I have never seen anything like the heavy duty gear the crane driver said it was 16 metric tons with 2 foot of water in the bilge and no fuel so when loaded 15 or 16 tons seems realistic .Without knowing what I am talking about i came to the same conclusion you did bigger prop or more pitch than a motor only boat.other motor sailers of the same length only weigh abot 9 tons and have half the hp.thanks John

[djmarchand]

Well, if I plug in 140 hp at 2,500 rpm, botdiesel's calculator says that the boat requires 71 hp at 2020 rpm to reach a hull speed of 8 kts. Since it is a displacement hull, more power won't push you any faster.

At 1,800 rpm the prop is pulling 51 hp from the engine and the boat is going 7.2 kts.

The prop required to do this is 23x14x3. That is a big diameter prop for a sailboat and I would bet you don't have the aperture for one that big. But that is what you get from the calculator with that big engine in a small boat.

What is the largest diameter that you can fit? Let's start with that and go from there.

David