Limited to 9.9hp

[oldboat1]

Re: Limited to 9.9hp

agree. It's just a hole shot that doesn't quite happen with the 9.8 -- ends up pushing a wall of water with the stern half of the hull. But if hull speed is increased more slowly (if rpms are increased more gradually), I have to believe the boat will remain relatively level and will get up to a decent speed. The boat looks like it's in nice shape, so have to believe the transom is straight and tight. (had a Lonestar with a Big Twin that did have some transom issues with the engine torque -- my fault in that case, not the boat's.)

 

[JimS123]

Re: Limited to 9.9hp

I finally found my Michigan Wheel catalog. For heavy loads they recommend a 9x7 3-blade prop for the 9.8. I personally think that prop with a DoelFin will be your best chance.

 

[slowleak]

Re: Limited to 9.9hp

I've got a 9x7 3 blade prop on the way, I just have to decide on which hydrofoil to try. I've got the following choices: Doelfin, Attwood Hydrolift, Stingray Jr, one unknown ribbed foil, and a super wide flat aluminum plate style foil.

The aluminum model loos too big and being that it's flat, it probably won't create much lift.
The Stingray is the thinnest plastic model, with the Attwood being only slightly thicker. Doelfin is the thickest.
The fact that the Stingray is one piece, it won't be as likely to damage the AV plate. The Doelfin looks like a one size fits all, they only make one size since its width is somewhat determined by lower unit size.
The Doelfin looks like it would create the most drag. The Attwood is angled upward and would most likely run well out of the water.

 

[ondarvr]

Re: Limited to 9.9hp

The shape of the fin (foil) has little effect on how it will work, other than creating drag when and if it's dragging in the water. The foil shape is more of a marketing gimmick than anything else. For just getting on plane a flat plate works just as well.

 

[slowleak]

Re: Limited to 9.9hp

The aluminum plate is 15" wide with a 1/2" rear lip turned downward with two 1/4" lips turned up on the sides.
It appears to hane been shaped to fit snug on a smaller motor.

I was thinking that the Atwood, whch is turned upward like gull wings, would get clear of the water faster and stay out of the water at cruise better on a heavy boat?

 

[oldboat1]

Re: Limited to 9.9hp

Here's what I mean: Ebay 131057431339. (Mine is similar. Important to have the articulated kind for control.)

middle seat. weight distribution..... (and best seat for fishing)

 

[slowleak]

Re: Limited to 9.9hp

The one I've got is aluminum and it snaps onto the end of the tiller like a socket on a ratchet wrench.
I really don't need a very long extension, I can almost reach it from the middle bench. About 15" or so would be all it needs. The problem is though there's not that much open water where I can just cruise with the extension, there's too many shallow spots and occasional other boats that require me to be at the motor to operate safely. In the lake there's about a mile long stretch from the ramp to where we fish that lets me just run wide open. Years ago with my old boat I'd lock the throttle, point the motor and stand up toward the middle and steer by shifting my weight, but that boat was a lot faster and easier to move around in with a flat floor.

 

[slowleak]

Re: Limited to 9.9hp

I got the three blade prop last Friday, this weekends warm weather was perfect to try it out, I also took along several of the fins to try.
First off, I ran it with the two blade prop with a Stingray Jr. fin, it sunk low into the water on take off and would begin to climb back up but then drop back down, and it did that over and over. I switched to the 9x7" three blade prop, this one an OEM Mercury prop that I found at a dealer out west. The 9x7 three blade prop without the hydrofoil eliminates almost all the sinking on take off, but it also didn't propel the boat well at all, the motor can over rev with that prop, I limited it to to 6000 RPM manually. I then added the hydrofoil again, and this time it would pop up on top of the water almost instantly, within about 15' or so, but I had very little speed, maybe 10mph or so, plus, everytime I hit a disturbance in the water or crossed another boat's wake, the prop would lose grip and slip.
The OEM prop has narrower blades and no hub flare compared to the MI Wheel 9x7 I already had which has wider blades and a flare on the hub where the exhaust exits.
I switched to the MI Wheel 9x7 prop and it drops the boat in a hole much like the 2 blade did, I then added a hydrofoil and that helps but the boat would pop up on plane pretty fast but then it sort of settles back in and only cruises along at maybe 9 mph, even slower than the Mercury three blade. The MI Wheel prop doesn't seem to lose grip as easy as the Mercury prop.
The 2 blade never loses grip with no hydrofoil but does when turning with a hydrofoil in place. I tried three brands of hydrofoils, Stingray, Doelfin, and Attwood, the Attwood affected the speed the least, the Doelfin and Stingray were about equal in lifting the stern, but the Stingray seemed to create less spray and turbulence behind the boat. The Attwood model, which is angled upward on both sides, got nearly completely out of the water but would settle back down till it was riding on top. I don't think any of the tests I did were a good comparison of the hydrofoils, but the Stingray seemed the best overall and it by far fit the best on the smaller motor.
The Doelfin seemed to lift faster and higher but its shape is more like an air plane wing then the others, giving it some natural lift plus the turned down rear edge. Being thicker, it probably has the most drag when in the water.

Nothing so far has ran as good on this boat as the 6hp did. I went back to the lake with the 9x7 Mercury prop on the motor and a different boat, I took a 12' Duranautic and tried that with the new prop and it was better, it didn't take on water, but it didn't go very fast and it wouldn't plane the boat. I took along a borrowed 9.5hp and it flew across the lake, even with me at the stern with a 130lb passenger up front. With just the 130lb driver, the 9.8 Mercury flew with the original 2 blade prop and the Stingray hydrofoil on the back. It would jump right up and take off. I tried the 9.8 with no other weight but myself in the middle seat and it half sort of got on plane but struggled on the edge of taking off like it should.
Today, I did one more test, I borrowed an older Polar brand modified V hull 16' boat from a buddy, its a bare hull, extra wide, which weighs in at 240lbs. Me and a 200 lb buddy took that out with the 9.8hp with the two blade prop and Stingray hydrofoil and it popped right up on plane and cruised along at about 14 mpg on his phone's GPS.
If we both sat side by side on the split bench in the middle, the boat would gain two mph.
He runs that boat normally with a 35hp Evinrude in the river. I think all I accomplished there though is convincing him to add a hydrofoil to his 35hp. I was temped to hang his 35hp on the Starcraft and try that combo but figured the weight would nearly sink the boat if it did the sinking trick on take off.

I also tried a 2004 Mercury four stroke 9.9hp motor which I borrowed, I'm not sure of which prop it had but on take off, it too sinks low in the water but not as low as the 9.8hp two stroke, but the difference is that the 9.9 four stroke pushed the boat at a pretty good clip even though it didn't get on plane. The resulting wake behind the boat was pretty bad. I didn't try that with a hydrofoil since it wasn't mine to drill holes in.
I didn't try the aluminum hydrofoil since it was so big and I'd have to cut and carve it up to make it fit the small motor properly. I figured it wasn't worth the risk of destroying it for something that may or may not work in the end.
I did see one boat running on the lake on Saturday that was running a 9.9hp Suzuki two stroke, the boat was about a 14' Mirrocraft with three grown men on board, the boat was on plane and traveling at a fairly good speed. I made a point of checking out the set up and that boat had two 13" long lengths of aluminum angle iron welded on both sides at the water line, flush with the bottom of the hull. Each angle was bent down just a bit. The guy said that without them the boat won't plane. Sort of a fixed trim tab. The angle was about 1 1/2" from the back of the boat. Of course, I'm not sure I'd be so willing to weld or rivet such a thing on the back of my super clean hull, but it did seem to work and it wasn't putting any stress on the motor's AV plate or clamps.

 

[starcraftkid]

Re: Limited to 9.9hp

It sounds to me like the 9.8hp is just too small.
I run into similar issues here on restricted lakes.
I find running a flat bottom boat helps and all of my 9.9hp motors are upgraded to 15hp.
A 15hp Evinrude will move along nicely on a 14' boat even with a maximum load.
I'm 300lbs myself and have never once felt that I was overloading my 14' boat.
Most of my fishing buddies are about my size, so the average load on my 14' Lowe is 700 lbs.
I have hit 23 mph on the fish finder but I doubt if that's accurate.

Since the two blade prop obviously is moving water, the issue has to be either weight or the hull, and since it moves ok with the smaller motor, that's telling me its a matter of the motor not having enough guts to push the boat up on plane, a lower pitch prop lessens the amount of water it can grab, but it also means the motor has to turn faster to do the same work. On an OMC motor, the changes to make it a 15hp motor do just that, they allow the motor to turn 500 RPM faster. On your 9.8hp, you really don't have that option. It is what it is.
They were good motors on light boats but not that great on a small boat. My gut feeling here is that you need a prop size and pitch that just isn't available for that motor.

 

[steelespike]

Re: Limited to 9.9hp

I think the 16 ft polar craft should tell you something.
It planed with you and a buddy and made 16 mph with the weight distributed right.
You need a no frills hull with a good footprint to get the weight up. The flattened V probably helps as well.

 

[slowleak]

Re: Limited to 9.9hp

I wouldn't mind owning that Polar Kraft I borrowed but he spent ten times as much on his boat than I did. I just don't have that kind of cash. It also didn't handle the weight as well, there was little freeboard if I moved about the boat, on my Starcraft, I don't think I can lean far enough to make the gunwales touch the water.

After the snow stopped today I did one more test, I hung the older 18hp on the 14' boat in the river, with a hydrofoil, with that motor, the boat flies, it never once dipped low in the water. However, its not legal in the lake. Without the hydrofoil, it would dip low, then gradually gain speed while rolling side to side a bit while it climbed on plane. The bow never did level off though.
The hydrofoil is required. A buddy brought his son along who weighs less then 100lbs, he also towed the Duracraft along. We hung the 18hp on the 12' boat and gave it a try, with me in the boat, I couldn't even move all the way back on the middle seat without it almost taking water over the stern. We put his son in the boat, and the thing flew, it bounced across the water like a skipping stone. The only bad part today was that I had to bring every motor we ran into the house afterwards so nothing froze up.

So far the best setup with the 9.8hp Merc on the 14' hull is the original 9x9p two blade prop and the Stingray hydrofoil. I think if I can find a prop in between the 9x9 two blade and the 9x7 three blade it would at least be tolerable.

 

[steelespike]

Re: Limited to 9.9hp

Your motors won't freeze if stored upright.
Here in the North east we sometimes boat into December.
I've seen our motor almost like a ball of ice from wind and wave spray when tied at the dock.We had to pour hot water over the the leg to thaw the ice before starting it.Sometimes had to chip ice off the steering cable.

 

[Sea Rider]

Re: Limited to 9.9hp

Tohatsu's has bit longer tails compared to other brands tails. Needs to sit at a proper transom height in order to work well, that's with well ballanced boat (cargo and passenger) trimmed to be at a 90? angle with respect to water level. Will need to make wot engine height tests with wooden shims of different heights untill sweet transom height is reached. Wake should be smooth with no over transom or back water splashes, if having them, that engine is badly height seated.

Anyway if the 9.8 HP can't plane that boat after engine hieight optimization, will push it much better with better prop thrust at fast displacement speed. This is the sweet transom ideal height for any Tohatsu horse :

Happy Boating

 

[gm280]

Re: Limited to 9.9hp

Knowing your lakes are posted to a max of 10HP engines, it doesn't say how many 10HP engines. So maybe you should put two 10HP OBs on your boat... :facepalm: If they catch you, you could always tell them one is just a backup incase the other one fails... :joyous:

 

[slowleak]

Re: Limited to 9.9hp

I thought about the two 10hp idea but that would mean at least 140lbs of motor hanging on the transom. Then there's trying to throttle and steer two tiller motors. I suppose one would need to be a tiller, the other a remote model cabled to the tiller steer model?

My motor isn't a Tohatsu, its a Mercury. I've tried raising it a bit, I can't go lower. It works best at the lowest height.
The wash behind the boat is huge, when topped out at full speed, there's two rollers forming on both sides, and a trough down the middle as big as the boat. I can watch the nearly two foot tall wake travel all the way across the lake.
Something I did notice with the new Mercury prop is that the hub 'tube' section is short, it exposes the entire prop nut and threaded portion with no flare. The MI Wheel 9x7 prop is flared and about 1.5" longer.

What does the flare on the prop hub do?

The one obvious improvement with the three blade prop is the ability to move better in reverse without having to hammer the throttle to make it move.

 

[H20Rat]

Re: Limited to 9.9hp

The part that gets me most is how much water the two blade prop seems to be moving, the wake and prop wash behind the boat is huge
I'm sort of thinking the situation here is sort of like a car spinning its wheels on a hill and digging a rut, the motor is working hard and doing its job but not getting anywhere. With this analogy in mind, it makes me think its a matter of the prop taking too big a bite too soon. On car, you have several gear ratios, on a boat, your always in high gear so to speak.
The prop seems to be moving water out from under the boat faster than the hull can progress forward against the now hill of water ahead.

It doesn't really work like that... The prop is actually moving very little water, under ideal conditions it would be somewhere around 10% prop slip. And any water you actually do move out from under the boat is being instantly replaced by new water, and the pressure difference because of Bernoulli's principle is very, very minimal.

 

[oldboat1]

Re: Limited to 9.9hp

think Bernoulli's principle would say something about the differing pressures of the moving water behind the boat, as opposed to the water under and ahead of the boat. I'm not sure how it would relate to movement of the hull -- except MAYBE, in keeping with what might be slowleak's intuition, the stern of the boat wants to slip into the low pressure area created by action of the prop. Which is to say, the boat wallows. This is heady stuff.

In keeping with slowleak's note (just the exerpt quoted in #77), the idea of shifting to a higher gear to overcome slipping wheels is interesting. Winter weather drivers understand that slowly starting in a higher gear can provide grip even with balding tires -- provided the throttle speed is low, so as not to defeat use of the lessened torque in the higher gear. Given the single gear in the outboard, as in Slowleak's analogy, it seems to me that the only way of overcoming friction of the hull is to back off the throttle until hull movement increases -- like the car's speed slowly increasing on the slippery surface. As long as the throttle is feathered, or at least not opened too quickly, the car's speed will continue to increase. So too with the boat.

 

[slowleak]

Re: Limited to 9.9hp

Slow and careful acceleration doesn't really seem to do the trick, when sitting still, in flat, open water, if I put it in gear, the boat starts to move ever so slowly, as I give it gas, the boat starts to move but starts to wallow in the rear pretty much in proportion to speed. If I run about 1/3 throttle, the boat just pushes water and keeps slopping water around out front, slightly faster and the bow starts to rise but the rear of the hull begins to drop. By the time I hit full throttle, the stern is below the surrounding waterline.
I have no doubt that a bigger motor is the real fix but that's not an option.
I think I need to find a 9x8p prop for this and run the hydrofoil as well.
I was also wondering if a good prop shop could modify the 9x7 prop to gain just a big of grip and keep the RPMs down.

 

[reelfishin]

Re: Limited to 9.9hp

Prop theory I think is a bit simpler than most think, a prop is a simple screw, which in application forms a simple pump to create thrust.
It is indeed pumping water from under the boat, then in turn producing thrust out the back. If the weight of the boat is creating more pressure than the negative pressure created by the prop beneath the boat, the boat will wallow.
The only answer is to either increase thrust, or lighten the boat. Since neither is likely possible since your on a restricted lake, all you can do is do the best you can to optimize the set up. First off, make sure the motor is achieving and not exceeding maximum RPM, second, once that is done and if it alone is not enough, then the addition of either a set of tabs or hydrofoil are the next and most likely the last resort. I've seen small homemade tabs made from simple aluminum angle iron mounted flush with the bottom of the boat, in effect, they do the same as a hydrofoil's down facing rear portion in that they force the hull upward. The advantage is that tabs don't put any added strain on the outboard or its mounting bracket. Both can affect top speed.

 

[oldboat1]

Re: Limited to 9.9hp

Like the idea of tabs (maybe those spring loaded types if they still make those). Just a very interesting thread (but hope you get a solution, slowleak).