prop question

[Priceless]

Fishing Saturday, 30 miles offshore and shaft snaps and prop bangs off hull. Luckily 31's are a little thick in the fiberglass so we don't sink. When repowered with Yanmar 315's in 2004 they used 1 3/8 shafts. Probably not sufficient especially after reviewing previous posts. Moving up to 1.5 inch shafts. Present props are 18 X 20 three blade. Worton Creek says they will put 20 X 25 three blades on their 315 repowers(they did not do the 2004 job, john said they always use 1.5shafts on 31's) but I am afraid idle speed will be too fast for low speed handling and offshore trolling. Trolling valves are not an option. Any members with Yanmar 315's and what props do you run? With my present setup idle at 600--5 knots, 1000-1100rpm--6.5-7 knots for trolling, 3100-26-27 knots and tops out 3850--33.5 knots with cruising economy of 11-12 gph. Pretty happy with my present setup but would like to here what you guys have to say

[Priceless]

Forgot to add-- running 1.5 to 1 gears

[Carl]

To keep slower idle speed with increased cruising speed you can keep the pitch down on the low side and add cup.

Cup does little is anything at idle speeds but acts like increased pitch at higher speeds. When adding cup you can cruise faster...and/or cut down on throttle some to increase economy or range.

[CHolgerson]

Hi Pat,

The reason Worton Creek's refits use bigger props is because they use 2:1 gears for their refits. You will be over propped if you try to spin a bigger wheel. I would recommend moving to a four blade DQX wheel. You should gain cruise speed and smoothness by running four blades. The boat was propped right and performs very well. I would not mess with it too much. I am glad you made it home safe.

The new boat has 1 3/4 shafts with big four blade wheels and 2:1 gears. Cummins 300hp

Regards,
Chris Holgerson