Honda 9.9 prop

[All U Get]

Hi all,

We were running the dinghy to a spot down the coast a bit and couldn't really keep up with the rest of the group. The problem was not being able to get the boat up on plane with our Honda 9.9 four-stroke outboard. Our 10 foot RIB weighs about 150 lbs. net and with the fuel, battery, and two of us add another 350 lbs. I have it trimmed 90 degrees to the water so the only issue I could see effecting performance was the prop.

The prop is a four blade aluminum 9 1/4 X 9 inch. Went to the Honda 9.9 site and the specs had a four blade but 9 1/4 X 8 inch. So the question is "Would replacing with a lower pitch (8 or less) get the boat up on plane?" I had done some searches but couldn't zero in on this topic yet I know I've read it on SBO.

All U Get

 

[Bill Roosa]

Lower pitch actually means higher gear. Only 8 revolutions per foot of theory movement forward vs 9 revolutions. So the motor will go slower for a given advance of the prop.
Can you max out the rpm on the motor? If so then going to a higher gear will probably help. If you cannot get to max rpm then going to a higher gear will not help and you are officially underpowered.

 

[kloudie1]

Yup, Like Bill says, if you are getting to full rpm, then ya need more pitch .. But..Chances are that you are not getting to full rpm hence not full horsepower.. so going to less pitch (8" "a lower gear") will allow the engine to spin up to more power and possibly plane the dink. Problem is getting a good RPM reading.. alternately find a place or a friend who'd let ya try the 8" to see if that solves the problem.

 

[Bill Roosa]

I'd say that you just need to embrace the horror of sail time and don't worry about keeping up.

 

[All U Get]

Lower pitch actually means higher gear. Only 8 revolutions per foot of theory movement forward vs 9 revolutions. So the motor will go slower for a given advance of the prop.
Can you max out the rpm on the motor? If so then going to a higher gear will probably help. If you cannot get to max rpm then going to a higher gear will not help and you are officially underpowered.

Bill that's different than what I remembered (being 1 revolution every 9 inches through the water). I was at WOT on the bay but don't know if I'm getting max RPM's. Since the outboard has an electric start, could I get a tachometer wired to it?

Kloudie1, I think you said the same thing but I got a little lost with the higher gear/lower gear comparison. Sorry for being a little like Bubba but is 8 inches a lower pitch than 9?

Now I read it over again and I'm thinking put the rum down. Lower pitch = higher gear = plane maybe. I've seen these 9.9's on workboats, skiffs, and trailer sailors so I know they have a good reputation. I just can't think of why the PO would have opted for this prop.

All U Get

 

[Johann]

Now I read it over again and I'm thinking put the rum down. Lower pitch = higher gear = plane maybe. I've seen these 9.9's on workboats, skiffs, and trailer sailors so I know they have a good reputation. I just can't think of why the PO would have opted for this prop.

All U Get

Perhaps the PO ran mostly solo and opted for the higher pitch for more top end speed. Can you get on plane with a lighter load, say just one of you in the RIB? If so, then barring any issues with the motor, I'd say you are over pitched.

How close do you think you are to getting on plane with your full load? If you are close to plane, maybe the 8" or even a hydrofoil...

 

[kloudie1]

So confusing sometimes.. Pitch number is how far (in inches) the prop moves forward when it makes one revolution.. with no slip. So a smaller number like 8 means more RPM for the same boat speed. which sounds wrong for your problem, BUT if you are "over propped", the 8" pitch will let the engine rev a bit more and produce more horsepower, which may allow it to plane the dink with both yall in there.
JoHann has a great suggestion.. if the dink will plane with one person, but not two, you are "over-propped" (too much pitch at 9") and going to the 8 inch will help.. but there is no way of knowing exactly if it will plane the dink with the higher load.
Think about climbing a mountain in a vehicle.. you may be able to make it (with 9" pitch) without going to a lower gear.. but if you are pulling a trailer, the engine will need to spin faster to make more power so it can get the bigger load up; you'll need a lower gear (8 inch pitch) to do that

 

[All U Get]

OK I'm going to see about an 8" when we get back home. With just me I can get it to plane at least some of the time.

All U Get

 

[Bill Roosa]

Let not forget we are on sail time here. 8 bells = 4 hours so a single bell is 30 minutes. Did you get there within 30 minutes of the last guy? If so then you are "on time" and don't worry.

 

[All U Get]

Let not forget we are on sail time here. 8 bells = 4 hours so a single bell is 30 minutes. Did you get there within 30 minutes of the last guy? If so then you are "on time" and don't worry.

On the two mile run we were about 20 minutes behind. Our plan was to hit one hour before high tide to get through the Shark River mangroves, snorkel for a while, then back through the mangroves before one hour after high. Kinda like Bogart and Hepburn going through with branches rubbing both sides and shading overhead.

The other dinghies have more HP, one having 40HP. I think we're the only ones with less than 15. There is quite a bit of turbulence at the prop when wide open so it appears to be straining to get going, just not doing it.

All U Get

 

[Parsons]

In addition to the horsepower, and in consideration of your experiment with planing with only one person, have you tried redistributing weight? Planing is as much a factor of bottom angle as it is raw power to get up on the back of your bow wake. I assume you tried with both of you sitting in the stern of the RIB.

 

[All U Get]

In addition to the horsepower, and in consideration of your experiment with planing with only one person, have you tried redistributing weight? Planing is as much a factor of bottom angle as it is raw power to get up on the back of your bow wake. I assume you tried with both of you sitting in the stern of the RIB.

We'll try this evening after the raft-up Sundowners.

All U Get

 

[Johann]

The other dinghies have more HP, one having 40HP. I think we're the only ones with less than 15. There is quite a bit of turbulence at the prop when wide open so it appears to be straining to get going, just not doing it.

All U Get

FWIW I have a 10' RIB with a 9.8 Tohatsu and with 350ish pounds of people/gear it will plane easily and make in excess of 15 knots, so I think you shouldn't have a problem with the right prop and load distribution.

 

[Bob 04 H260]

I have a suggestion, make sure the tubes are rock hard, this does make a big difference even on a rib. The boat will sit a bit higher in the stern, makes it easier to get up on plane.
I think your prop is aluminum. I had an aluminum prop repitched here in Stuart for $32. Drop it off at 9 am ready at noon, Stuart Propeller if you are coming back north this way.

Bob

 

[All U Get]

I have a suggestion, make sure the tubes are rock hard, this does make a big difference even on a rib. The boat will sit a bit higher in the stern, makes it easier to get up on plane.
I think your prop is aluminum. I had an aluminum prop repitched here in Stuart for $32. Drop it off at 9 am ready at noon, Stuart Propeller if you are coming back north this way.

Bob

Thanks Bob, yep on the aluminum. We haven't settled on where we would enter on the way back. Weather would make a big difference.

All U Get

 

[Bob 04 H260]

We plan to cross this Tuesday from Lake Worth inlet to West End then on to Marsh Harbour over the rest of the week. Looks like we might be delayed in Green Turtle for good Whale crossing ! We will have to see, not a bad place to wait if you like Miss Emily's Goombay Smash !
Bob

 

[jibes138]

Make sure the bottom of your dinghy is clean. Any growth will cause drag and might keep you off plane. Sometimes if you move someone to the bow to get you on plane then you can move back. You may just have too much weight in the stern, gas, battery, driver, etc. Maybe try adjusting your motor trim, you noted it is 90 degrees I assume at rest but once the bow comes up you are no longer at 90 degrees.

 

[Gunni]

I switched from the Tohatsu9.8 4-stroke OEM 3-blade 9" pitch prop to their "high thrust" 4-blade 7" pitch prop and find that the boat comes up on plane quicker, and will plane with a load at a lower motor speed, and seemingly at a slower boat speed. Top speed also seems to be reduced, but I don't care. Overall, a good modification for your hauler.

 

[All U Get]

Make sure the bottom of your dinghy is clean. Any growth will cause drag and might keep you off plane. Sometimes if you move someone to the bow to get you on plane then you can move back. You may just have too much weight in the stern, gas, battery, driver, etc. Maybe try adjusting your motor trim, you noted it is 90 degrees I assume at rest but once the bow comes up you are no longer at 90 degrees.

The bottom was cleaned three weeks ago and a little wax too. I've pumped the tubes hard and we've adjusted trim and weight. I'm leaning toward the prop pitch and the Brig dinghy design. I had it plane today with only me aboard so the 7" pitch would probably do it with both aboard. Another cruiser here had done that to his outboard and gets moving pretty fast.

All U Get