Prop size

[AbeFroman]

I have a 1985 international series 10 m sedan . It is powered by twin crusader 454 CID motors. My gear ratio is 1.91 to 1 and I am currently running 18 x 19 three fin props. At 3000 rpm I should be running a little over 16 knots but am only running about 14. I am also not getting the fuel efficiency I would like or the range that I should be achieving. I thought perhaps I should change props but the prop calculator that I used told me I needed to run 21 x 24 which is a huge jump. I'm afraid of straining my motors and wasting money even purchasing props that big. Does anybody have any advice on the correct prop size for fuel-efficiency and performance?

[AbeFroman]

Also I'm not even sure if I have enough clearance to run a 21 inch prop.

[prowlersfish]

There is no way you could turn 21x24 props it would be straining / lugging the engines badly. What rpm are you getting with your current set up at wot ? Your prop calculator is wrong , remember a prop calculator is only as good as the info put in .

[AbeFroman]

I am turning 3900 RPM at wide open throttle running about 19 knots. I usually cruise at 3000 rpm doing 14.5 knots. Both of those speeds seem a little low to me. I have a hard time believing that is performing at this boats full potential. Am I wrong?

[AbeFroman]

I am turning 3900 RPM at wide open throttle running about 19 knots. I usually cruise at 3000 rpm doing 14.5 knots. Both of those speeds seem a little low to me. I have a hard time believing that is performing at this boats full potential. Am I wrong?

[prowlersfish]

look at this thread , http://www.trojanboats.net/wforum/viewt ... 8&start=15

[WayWeGo]

While not the same hull, this may help...

F36 with 1:91 gears running 18.5x19 three blade props. With a clean bottom at 3,750 RPM (had a throttle cable adjustment issue), she ran 25.5 knots at sea trial. At the end of this season, she ran 23 knots into a 20+ knot headwind at 3,900 RPM after sitting in the slip most of the season with only a couple of times out. The bottom was cleaner when I got back to the slip; the bottom paint was done just before launching in the spring and I had sanded the hull down to the barrier coat so it was pretty smooth before painting.

A funny story about that headwind. On the way out, we had the wind on our starboard beam and on plane, the boat was heeled over to the port so hard, I could not level the boat with the trim tabs. I had to stop to inspect the tabs and make sure both props were being driven before I accepted the wind was so strong. I guess having full canvas on the flybridge really does act like a sail in heavy weather!

I have fuel flow sensors still sitting on the bench, so no usage or efficiency data.

[prowlersfish]

The F36 conv/sedan is a faster hull then the 10 meter sedan as it has less V.

[WayWeGo]

I pretty much assumed that the 10M would be a bit slower than a F36, but do you have any idea how much slower? Did the delta conic hull help much with fuel efficiency?

[prowlersfish]

I pretty much assumed that the 10M would be a bit slower than a F36, but do you have any idea how much slower? Did the delta conic hull help much with fuel efficiency?

Slower with the same power = burning more fuel . I don't have any hard numbers but they 10 meter sedan is slower then the express and mid cabin . I am sure this is due to it being a heaver boat . My late best friend Wes had a 10 meter fly/aft cabin . And while it was a great boat , it was slow and burned a lot of fuel (.5 mpg on a good day). It was heavy but rode great .