Questions on getting to know my new Tohatsu 9.9EFI

[Quad]

Hi all - first time posting here, interested to hear some of your thoughts.

I've got a new RIB - a Coastal Inflatable Boats M-RIB 10ALU paired with a Tohatsu MFS9.9ES EFI outboard motor with the stock 9.2" diameter, 9" pitch, 3 blade prop. I went with the 9.9HP motor so that my son can use the boat between now and when he turns 12. The motor starts amazingly easy and purrs like a kitten.

Overall, the setup works pretty well, however getting on plane with 2 adults has proven to be a challenge. Alone or with my son, we can get on plane and cruise at 16-17mph. I do observe that the note of the engine does not change much past half throttle. I have a tachometer arriving today which can help me understand where I am in the 6000RPM band and based on those findings, I am considering re-propping the motor with a 7" inch pitch prop. If I go this route, I have the option of a 10" diameter, 4 blade prop or a 9.2" diameter, 3 blade prop.

What should be the deciding factors here? I would really like to be able to get on plane when my wife is with us and cruse at speed vs the putting along at 6mph while we displace water.

Also, is there a way to read hours on the newer Tohatsu portable outboards? I bought this motor brand new, and want to ensure that I am not stepping out of bounds of the break-in parameters.

Thanks!

[Limecc]

Originally Posted by Quad

Hi all - first time posting here, interested to hear some of your thoughts.

I've got a new RIB - a Coastal Inflatable Boats M-RIB 10ALU paired with a Tohatsu MFS9.9ES EFI outboard motor with the stock 9.2" diameter, 9" pitch, 3 blade prop. I went with the 9.9HP motor so that my son can use the boat between now and when he turns 12. The motor starts amazingly easy and purrs like a kitten.

Overall, the setup works pretty well, however getting on plane with 2 adults has proven to be a challenge. Alone or with my son, we can get on plane and cruise at 16-17mph. I do observe that the note of the engine does not change much past half throttle. I have a tachometer arriving today which can help me understand where I am in the 6000RPM band and based on those findings, I am considering re-propping the motor with a 7" inch pitch prop. If I go this route, I have the option of a 10" diameter, 4 blade prop or a 9.2" diameter, 3 blade prop.

What should be the deciding factors here? I would really like to be able to get on plane when my wife is with us and cruse at speed vs the putting along at 6mph while we displace water.

Also, is there a way to read hours on the newer Tohatsu portable outboards? I bought this motor brand new, and want to ensure that I am not stepping out of bounds of the break-in parameters.

Thanks!

Hi Quad and welcome to the forum.

You're doing the right thing getting a tach.
A larger dia prop and a 4 blade at that will have much less slip, more grip and the lower pitch is more suited to a loaded boat. You should optimise the engine height and trim and get a benchmark as a first step, members will advise. Mount the engine as high as possible without the prop ventilating, use different thicknesses of timber on the transom to test.

When you've found the best propeller you could also consider fitting a Doel fin which will reduce the effort/speed needed to plane.

When the engine's out of warranty or if you choose to forego it you can unleash 20hp out of this EFI engine with a few parts fitted by any competent diy'er. Then you'd have no planing issues You'd also need a 20hp prop.
Post #19 refers to Suzuki but similar for Tohatsu/Mercury: Upgrade motor or new boat?

[Fenlander]

Yep welcome to the forum.

Is your boat a true RIB with alloy hull or a SIB with alloy floor and fabric outer?

A tach will give some clues but its reading is only useful when on the plane and will tell you little when you're held to displacement speed.

A sensitive topic I know but what sort of weight do the two adults total? We are about 25stone between us and our experience with SIBs from just over 3m to 3.8m is that we can plane easily with 10hp two up making 15-17kts depending on which boat. This experience is with a 4-stroke 9.8 and two examples of a 2-stroke Tohatsu 9.8. That is on stock props of 8.5" pitch.

But assuming you have inflation pressures and trim sorted than it does look as if you need to drop a pitch. A standard design 3-blade of a lower pitch would be the easist to judge but choosing 8" or 7" is a bit of a guess. There are calculations that should predict the result but they are not accurate compared to the actual result you might see.

Just a warning changing prop brands or diameter/no of blades will make the choice less easy because the results may not be in line with a simple pitch change on a standard 3-blade design.

[Quad]

Originally Posted by Fenlander

Yep welcome to the forum.

Is your boat a true RIB with alloy hull or a SIB with alloy floor and fabric outer?

A tach will give some clues but its reading is only useful when on the plane and will tell you little when you're held to displacement speed.

A sensitive topic I know but what sort of weight do the two adults total? We are about 25stone between us and our experience with SIBs from just over 3m to 3.8m is that we can plane easily with 10hp two up making 15-17kts depending on which boat. This experience is with a 4-stroke 9.8 and two examples of a 2-stroke Tohatsu 9.8. That is on stock props of 8.5" pitch.

But assuming you have inflation pressures and trim sorted than it does look as if you need to drop a pitch. A standard design 3-blade of a lower pitch would be the easist to judge but choosing 8" or 7" is a bit of a guess. There are calculations that should predict the result but they are not accurate compared to the actual result you might see.

Just a warning changing prop brands or diameter/no of blades will make the choice less easy because the results may not be in line with a simple pitch change on a standard 3-blade design.

The boat is a True RIB - it has a welded aluminum hull/transom and a welded-in flat deck.

I don't mind discussing the weight factor - I'm at ~180lbs, wife is at ~125lbs, kiddo is around ~85lbs. I also keep 100lbs of sand in the bow locker and I can get on plane barely with myself and kid along with that sand. With all 3 of us humans and no sand, we can't quite get there. It's VERY close, especially if we shift weight around, but it's just not enough.

I do know for a fact that we were underinflated for the first 2 weeks we had the boat. I did not yet have a pressure gauge, and we went by feel. Turns out we were hardly registering any pressure and with the gauge, I got all chambers to the specified 0.25bar/3.6psi.

Regarding the prop change, introducing as few factors as possible has me leaning toward the 3 blade design with the same diameter. I was planning to purchase an OEM Tohatsu prop to reduce variables as well - the 3 and 4 blade options are both OEM.

[Fenlander]

OK so you are about my weight and your wife lighter than mine. Your kid is near enough to my 8 stone daughter.

For you and the kid I'd be leaving the sand behind and with him/her up forward you should pop on the plane easily. I would have thought you and the wife would have been on the plane fairly easily too with no sand but understand from experience how that third person even if a child can make things a lot more sluggish with a 10hp motor.

I'm not saying you should do this because I don't want to waste your money in case it doesn't work but I'd cross my fingers and just drop to an 8" OE 3-blade. Personally I've never had to pitch down as low as a 7" with a 10hp.

[Quad]

Originally Posted by Fenlander

OK so you are about my weight and your wife lighter than mine. Your kid is near enough to my 8 stone daughter.

For you and the kid I'd be leaving the sand behind and with him/her up forward you should pop on the plane easily. I would have thought you and the wife would have been on the plane fairly easily too with no sand but understand from experience how that third person even if a child can make things a lot more sluggish with a 10hp motor.

I'm not saying you should do this because I don't want to waste your money in case it doesn't work but I'd cross my fingers and just drop to an 8" OE 3-blade. Personally I've never had to pitch down as low as a 7" with a 10hp.

Good perspective on 8" vs 7". Once I've got the tachometer readings, I hope to have a better feel for where I need to go.

[Quad]

So - I've re-propped with an 8" pitch, 3 blade.

With myself an 100lbs of sand in the bow on board, I can get my engine RPMs to 5500-5600 (up from 5200). This puts it in the power sweet spot. It hops on plane more quickly and there was no sacrifice in top speed. Now to test with the wife and kid!