Getting it up--optimizing trim with propeller choice?

[DARice]

Short version:  Can get my bow higher at cruising speeds with a different propeller? If yes, what are the tradeoffs?

More of the story...
Unlike most 17s, at rest my trim is more level, just a couple of inches lower at the stern. There's no secret to that, I have more weight forward than most: my boat came with batteries in the peak of the cuddy, I have a drum winch on the bow, and now a pilot house.

The boat performs very well, speed and efficiency are very good, as is handling. Even before the PH, I never had an issue getting the bow down once on plane, and time to plane has always been fast.

Since adding the PH I've gone from an Amita 4 13X15 being my preferred prop to an Amita 3 13.5X15 that has less stern lift. Qualitatively, I still prefer the cruising handling of the boat with the Amita 4--it feels 'sportier', almost like the entire hull is higher in the water--but everything's a tradeoff.

Performance goal: be able to get the bow up higher while on plane at moderate speeds (overtaking following seas), but keep it down in most conditions.

This is fine tuning performance for specific conditions that can be managed in other ways, so I don't want to:
-reconfigure the boat, for now (move batteries, replace with light weight, high $$ batteries,...)
-introduce new performance issues (e.g., the need for trim tabs to get the bow down).

Dave
p.s. Yes, it's raining and it's been a couple of weeks since I've been able to take the boat out.

[Tom C]

Remind us of the boat's top speed and RPM with the 13" x 15" Amita 4 on your 2021 Honda BF90

[DARice]

Remind us of the boat's top speed and RPM with the 13" x 15" Amita 4 on your 2021 Honda BF90

I don't have #s with the Amita 4 since adding the pilot house.

With the Amita 3 13.5x15 in fresh water, 2-5 mph wind, ~70 degrees. 18 gal fuel, light load.

RPM, MPH
3500, 13.6
4000, 20
4500, 24
5000, 28
5500, 30.5
6000, 34, also WOT (trimmed up to porpoise, then brought down to record #s)

With the PH, I dropped down ~100 RPM using the Amita 3. Prior to the PH with the Amita 4 13X15 and lightly loaded RPMs would reach 6,300 in ideal conditions trimmed just short of porpoising. Also, with the 4 blade, performance was less sensitive to load.

Dave

[Tom C]

Well, you can try the 14" x 15" Turbo Pontoon I have. I actually think it might be a bit too much pitch, but you never know. It's in Seattle.

[DARice]

Well, you can try the 14" x 15" Turbo Pontoon I have. I actually think it might be a bit too much pitch, but you never know. It's in Seattle.

Yes, probably too much diameter and pitch for the motor/boat combination. With the small amount of slip that I have, a 14p seems doable, or a smaller diameter 15p.

Do you think that I'm chasing an objective that's unreasonable--having enough trim range on plane to get the bow way up, or keep the boat flat, depending on conditions? Or that changing the weight distribution of the boat is a better solution, or at least part of it?

Dave

[Fisherdv]

Well, you can try the 14" x 15" Turbo Pontoon I have. I actually think it might be a bit too much pitch, but you never know. It's in Seattle.

Yes, probably too much diameter and pitch for the motor/boat combination. With the small amount of slip that I have, a 14p seems doable, or a smaller diameter 15p.

Do you think that I'm chasing an objective that's unreasonable--having enough trim range on plane to get the bow way up, or keep the boat flat, depending on conditions? Or that changing the weight distribution of the boat is a better solution, or at least part of it?

Dave

If main concern is "not being able to get it up", no pun intended, I think weight distribution may be a good place to start from what you've described. Sounds like it's to easy to get the bow down, but not able to trim bow up high enough. With the weight of the PH, you should be able to move the batteries to the back. At least give it a try and see if that helps.

[davidsea]

 
 Without having to rewire anything to check your weight distribution, disconnect both batteries and move them aft, more or less where they'd go if you installed them there.  Use a pair of long jumper cables to hook up the start battery to your existing cables in the cuddy.  This may help - or not - but at least you'll know for sure, and the swap won't take much time.
   I've posted this before for people wanting to move weight forward:  Buy 2 bags of pea gravel at Home Depot, and put them forward where you're thinking of moving weight.  (Each bag is roughly the same weight as a GP27 battery)  If you don't like the results, cut them open and dump the gravel overboard - your experiment only cost $12...

[Tom C]

Do you think that I'm chasing an objective that's unreasonable--having enough trim range on plane to get the bow way up, or keep the boat flat, depending on conditions? Or that changing the weight distribution of the boat is a better solution, or at least part of it?

A better prop will certainly help you achieve your goals, the challenge is figuring out which one will work.

Do not make the common mistake of thinking propellers can simply be described by diameter and pitch, there is far more to it than that.

I mention the Turbo Pontoon because it was so succeful on both Dai20bt's and Scaleymojo's boats. It is an odd propeller, obviously designed for pontoon boats, but Yamaha discovered it was great in other applications as well. The grip of a four blade with the speed of a three blade, vastly better reverse thrust too. But they never made it in a 14" pitch version, which is what would work best for your boat.

That leaves us trying to find another model that grips do well that it will support the bow when the motor is trimmed out. No aluminum prop will really do this; you need the substantial progressive cupping of a good stainless prop to accomplish that.

Making things harder is the fact that your's is an intermediate size gearcase for which the propeller model options are more limited, especially even trying to find something with a lot of blade area.

I sold a few Stiletto Bay Pro props back when I was a dealer for PPI, and while that brand name was retired long ago, Yamaha still manufactures it as the Turbo FX4. There are also new-old-stock, and used deals that pop up now and then.