Prop question for new Honda BF250 on 25 foot Cobia

So, took my Cobia 250 WA out for the first time yesterday to break in the new Honda BF250 motor.

Hole shot seemed pretty good and got up on plane quickly.

I'm not the best yet at proper trimming, but felt I was about in the right place.

At WOT in slightly choppy seas I was getting max 6000 rpms and 39.5 MPH according to GPS.

Granted, this is a brand new engine and I assume it may get better with age after all broken in, but I'm wondering if my prop isn't the one it really needs.

Specs:

Length at around 25 feet, but add 2 feet for bow pulpit

Has hardtop which may contribute to a little loss of speed

Beam is 8'6"

Weight dry is 3750

138 gallons of fuel (I had full fuel tank yesterday)

Weight of Honda motor is 622 pounds, has 30" shaft

Two Group 24 1000CCA batteries in rear transom box

Two people on board total weight for both of us about 425 lbs

10 gallons of fresh water in berth area

Current prop that the dealer installed with the Honda is a Solas SS 14.25x17 3-blade

I have a spare aluminum prop from my KeyWest that is a 14.5x19 3 blade and was wondering if I could use it - maybe try it out with the higher pitch and see if it makes a difference.

Otherwise, what would you recommend for a prop? Speed isn't important as much as efficiency at cruise. I like to get good MPG. My use is mostly going to be cruising at around 28-30 mph, 90% of the time it will be just me, the wife, and fishing gear. But there will be times when I'll have 4-6 people on board with at least 3 of them having dive gear, two tanks each, etc.

Thanks for the help!

Re: Prop question for new Honda BF250 on 25 foot Cobia

The first thing you need to know is what Honda's recommended maximum rpm range is for that motor. If you're exceeding it at WOT you need more pitch. If you aren't making max rpm's you need less pitch. In short, once you prop for WOT the rest falls into place.

Once you've calculated what pitch will put you 'in the zone' consider a 4-blade for good mid-range and adjust your pitch requirement accordingly; and also according to which material you want to use (stainless vs aluminum).

Re: Prop question for new Honda BF250 on 25 foot Cobia

Full Throttle RPM Range 5300 - 6300 RPM

Rated Power 250 HP @ 5800 RPM

Gear Ratio 2.00:1 (24/12)

JoLin said:

The first thing you need to know is what Honda's recommended maximum rpm range is for that motor. If you're exceeding it at WOT you need more pitch. If you aren't making max rpm's you need less pitch. In short, once you prop for WOT the rest falls into place.

Once you've calculated what pitch will put you 'in the zone' consider a 4-blade for good mid-range and adjust your pitch requirement accordingly; and also according to which material you want to use (stainless vs aluminum).

Click to expand...

Re: Prop question for new Honda BF250 on 25 foot Cobia

Okay, so you're at 6000 rpm for a motor rated between 5300-6300 rpm. If you did your tests with your typical load you have the right pitch. If it was unusually light you could afford to drop one inch of pitch w/o overrevving the engine.

If you go to a 4-blade prop in the same material (I don't know what yours is), you'll want to drop one inch of pitch (from 17 to 16). If you can't find a suitable prop in 16-pitch you could probably afford to stay with 17 as you're already running at the high end of your rpm range. Staying with 17-pitch will put you closer to the middle of the range- say 5800-5900 rpm at WOT.

If you're moving from an aluminum prop to a stainless prop, you want to drop an additional inch of pitch because stainless doesn't flex as much as aluminum.

Hope this made sense.

Re: Prop question for new Honda BF250 on 25 foot Cobia

Your 2.0 gear (lower shaft speed) ratio favors more diameter.

Your dealer should have put the Solas Rubex (Lexor) 15.5 X 17 prop on there.

It would be running 200- 300 rpm less at any given speed, providing stronger acceleration and more economical cruising performance.

Re: Prop question for new Honda BF250 on 25 foot Cobia

I am using a Solas SS prop - my boat guy (who races high-speed boats) said we may be almost right in what I have, but perhaps may need to move the engine up a hole on the mounting, and go to a 4-blade. I'm hoping that we'll soon be able to get out on some calm water and open her up and see - we have a lake nearby that is ideal for this, but again I've got a full fuel tank (138 gallons) and may not have been ideally trimmed.