Prop Change for 20 More HP?

[cp]

I'm planning to upgrade from an old Honda BF130 to a newer BF150 and am wondering what size/pitch prop I will need for the new higher HP motor. The prop on the old 130 HP motor is an aluminum 13.75"x13p 3-blade and is sufficient to attain WOT 5700 RPM, well within Honda's specified 5000-6000 WOT RPM. I will be keeping the same boat, just putting the newer 20-HP higher motor on it. I'm concerned that the additional 20-HP will result in over-revving the motor if I use the old prop.

So, if any of you out there have upgraded to a 20-HP higher motor ON THE SAME BOAT, please tell me what prop you had on the old motor and what prop you now have on the new one and what WOT RPM each attained. Please be specific regarding material, diameter, pitch, and blades.

Any info from other repower 20-HP (approximately) increases on the same boat, like from a 175 HP to a 200 HP, or a 200 HP to a 225 HP, will also be helpful. I just want an idea of what prop change is necessitated by a 20-HP increase. I can guess-timate the RPM change from a diameter or pitch change once I get in the ballpark using any info from similar 20-HP upgrades on the same boat. I emphasize "on the same boat" because comparisons between boats with different hulls are not valid for my purpose.

Any input will be appreciated!

 

[QC]

Re: Prop Change for 20 More HP?

You need to consider gear ratio in this discussion . . . It's not just 20 hp, it's 20 hp with what gear ratio. I am seeing the opposite of what I woudl've expected. Early 2000s BF130s were 2.00 and the later BF150s are 2.14, so this is kind of screwy. Can you verify what you have and what you're getting as far as gear ratio?

 

[steelespike]

Re: Prop Change for 20 More HP?

I have seen the same ratios. I'm guessing their looking for a little better low end punch.
I say run the same prop and make changes based on the results.you can run a brief overev
to get soime figures.
Apparently the boat is a secret.lol

 

[cp]

Re: Prop Change for 20 More HP?

QC - The gear ratios you list correspond to what I know of the motors, i.e. 2.00:1 for the old BF130 and 2.14:1 for the BF150.

Please elaborate on what you are seeing, i.e. what is opposite of what you would've expected and what prop did what. Thanks for your input.

steelespike - I don't think the boat matters since it'll be the same, just with 20 more horses pushing it, so I want to keep the question clean and simple so responders won't go off on a tangent.

I can certainly run the new motor with the old prop to get a baseline, but am looking for input to help cut short that trial-and-error process. Days on the water here are better spent fishing than testing props.

 

[QC]

Re: Prop Change for 20 More HP?

Please elaborate on what you are seeing, i.e. what is opposite of what you would've expected and what prop did what.

Typically as horsepower goes up the numeric gear ratio goes down. So let's say it went to a 1.80, then it would be possible that you could use the same prop as the ratio alone makes up for the increased power. Prop will spin faster at the same RPM because the ratio is "faster". In your case horsepower goes up, and the ratio goes up, so it is more likely you will have to add pitch. Sort of a double wammy. Will this be a 2010 BF150? What year 130? Just want to add as much clarity as we can, which brings me back to the boat. Don't assume that secrets help with clarity . . . More info = mo betta answers. I'd like GPS speed at WOT, boat info and the motor year stuff.

Edit: I see you're at 5700 RPM now, and with the added 20 bhp plus the ratio issue, I'm going to make a wild arse stab that you're gonna need to go up one pitch. If you are satisfied with a typical three blade Aluminum go same to a 15. She may still lose a little RPM, and as long as your hole shot is good now I wouldn't be that concerned. Potentially a little better fuel efficiency at a lower WOT RPM too, especially with 4 cycles. You have a 1000 RPM to play with so not that bigga deal. If you want da kine, you're gonna need to test, test, test. Your boat and your motor, and your load and your needs determine wassup. All the udda stuff is nice, but you need data from your setup to make definitive decisions.

 

[mommicked]

Re: Prop Change for 20 More HP?

I lke "mo betta"

 

[steelespike]

Re: Prop Change for 20 More HP?

I don't claim to be an expert and the prop size and rpm does give some clue to the boat type
Without speed the rpm doesn't mean much.And while the typical rule of thumb does apply to pitch change rpm I'm sure the motor would react differently on a non planing hull, a pontoon for instance,or a house boat that may allready be at its ideal designed top speed.I'm sure prop results would be different on an easily planed hull.

 

[QC]

Re: Prop Change for 20 More HP?

His comments about fishing and location of Hawaii told me some sort of planing fishing hull, prolly a Center Console, but I understand your points completely ss

 

[Tail_Gunner]

Re: Prop Change for 20 More HP?

Bingo

ESSENTIAL DATA REQUIRED
When posting a question about propellers, you cannot get a reasonable answer unless you provide the following information in your post: Items 1-4 are essential for meaningful response to most questions.
1) Performance issue you are trying to correct.
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
3) Current prop diameter and pitch (required).
4) Wide open throttle RPM and speed with an average load (very helpful)
5) Engine/drive make, model, year, and HP
6) Boat make model, year, length and weight

Current specs....20hp is very little in marine terms

​

 

[cp]

Re: Prop Change for 20 More HP?

QC - Again, thank you for your informative reply. Thank you also for actually reading my initial post, which contained data from my present setup. I will start with a 15p prop and adjust from there.

Tail_Gunner - My initial post: (1) states that my only "issue" is wondering what change in prop might be necessitated by a 20-hp increase; (2) & (3) & (4) provides my existing prop info and WOT rpm; and (5) gives engine info.

I did not include any info on the boat or WOT speed because I wanted to focus on just the 20-hp increase without getting sidetracked by issues regarding top speed and sufficient horsepower, etc. Here in Hawaii, where deep water is right offshore and the trade winds generally keep the seas rough, we have no need to run long distances to reach the fish nor do we run at WOT, or even on plane, for any significant length of time hence WOT speed is irrelevant and there is no need for huge or multiple outboards, as I commonly see on boats off the East or West coasts.

So anyways, now dat QC went gimme da good scoops awreddy, below get da info all da niele guys like know mo' betta for dey can make olelo wehewehe. And I stay rite um in da King's English cuz bumbye dey no can read the Queen's (Liliuokalani) kine. But ain't no big ting, cuz stay pau.

My boat is in fact a Boston Whaler cabin model, a Conquest 21, and the BF130 is sufficient to get it on plane and cruise at 18-19 knots (GPS) with WOT speed of 25 knots (GPS). This is with a full load of fuel, three or four fishermen, ice, tackle, and sometimes even fish ;-). In over a decade, I can count on one hand the times we've been able to run on plane continuously for more than a couple of minutes. Those couple of minutes at the end of nearly every trip are when coming into the channel to the harbor and serve to beat the breakers and blow some carbon out, but mainly to verify that everything is working well as indicated by being able to smoothly attain the 5700 rpm WOT. I expect the BF150 with the appropriate prop will do a little better and have better fuel economy to boot.

Mahalo and aloha.

 

[steelespike]

Re: Prop Change for 20 More HP?

Knowing the boat and wot and rpm tells us if the setup is working. And what results a prop
change might have.I think its pretty obvious that you're rpm change with your boat isn't as likely to be as much as that on a light fast hull.I still say run the present prop and base the change on the results.