Boat won't plane out?

[crackedglass]

First off, let me describe the boat, it's a 14' Starcraft the bench seat aluminum boat, with a pair of clamp on fold down seats and a mercury 9.8hp outboard.

With two guys in the boat, no other gear besides a battery and 30lb thrust stern mount trolling motor, and a 3 gallon gas tank this boat won't plane out. It almost gets there but not quite.

I've tried it with on person, no trolling motor and only a gallon of gas in the tank and it makes no real difference. The motor is strong, has 137 PSI on both cylinders and runs like new. The boat is super clean, no excess weight or wet foam in the seats. The motor is trimmed all the way in.
The motor has a two blade prop, the one it came out of the box with, and seems to pull pretty hard. The boat leaves an unbelievable wake behind it under power, the motor is obviously pushing a lot of water. Top speed on the GPS is about 8 mph or about a slow walking pace. I had the same motor years ago on a modified V aluminum bass boat that did much better.

Weight seems to make little difference, even with my me in the boat, it's slow, not much more than a mile or two per hour difference. I weigh about 325, my fishing buddy is about 250 or so. The boat is restricted to no more than 9.9hp max by state law so hanging a bigger motor isn't an option.
We had a 16' boat but that wouldn't even try to plane, so I picked up this hull, what I believe is a 1973 Starcraft Seafarer. I had this same motor on a 12' jon boat and it was scary in that the bow rose up and we had little control, but on the V hull Starcraft it feels lame. The boat is light, two guys can car top it if need be, I'd guess about 450 lbs or so tops.

Would a prop change maybe help???

 

[5150abf]

Re: Boat won't plane out?

Move your battery up front and move the motor up a pin or 2 and see what that does.

You are kinda of asking alot though, that is alot of weight in a small boat and not alot of power, shifting weight and trim may be the answer.

 

[DonHof]

Re: Boat won't plane out?

It all comes down to weight distribution, move things around till you find out what works. I have a 15 foot with a 30 hp motor, I put a trolling motor battery in the back and it would not plane.

 

[roscoe]

Re: Boat won't plane out?

Uhhh, too much weight in the boat. Never gonna get it to plane with 500-750# in it with only 10 hp.

 

[Stachi]

Re: Boat won't plane out?

what is the weight capacity of the boat ?

 

[jdsgrog]

Re: Boat won't plane out?

yeah, sounds like too much weight.

 

[crackedglass]

Re: Boat won't plane out?

CG plate says 1120lbs.
I'd guess that it's about 2 mph from getting on plane. The motor isn't screaming either, its lugging a bit, which makes me think a prop change may help?
There's not a lot of options here, I can't go to a smaller boat, the law says the motor can't be over 9.9HP, and the boat is light. The battery is mounted under the middle seat, the fuel tank is just behind it. Moving the battery all the way up made the motor blow the prop out at wide open throttle. With the motor tilted out, the bow raises way up, it's on the furthest inward hole now. The anti-cavitation plate is 1" above the bottom of the boat and the motor is a clamp on short shaft with manual start. Not much changes when I leave the trolling motor, battery and second passenger on shore, but with just me in the stern, it actually seems worse. I tried both a 12 and a 16' hull too, the 12' couldn't handle a second man due to weight, the 16 would barely move, maybe 6 mph tops with the same motor. I've tried a buddies motor, a 15hp Evinrude with a 9.9 hood, and it's not much better, it goes a bit faster but still not really on plane, more of a stern heavy plowing action.
Even with the motor in the furthest forward hole, and the second man sitting all the way forward, the boat still rides bow high and makes a HUGE wake. I've even tried a tiller extension and sitting on the middle bench but that don't make it much better, just ads to the loss of bite. I've tried this with both mine and my buddies 15hp.
I moved from a 14' flat bottom jon boat that had capacity issues and couldn't handle the 9.8hp motor to this boat with much taller sides. It's rated at 20hp max, but this state don't allow it other than in the river. With a 20 hp on it, it flies, its downright scary on open water but I can't pass a 20hp off as a 9.9hp motor. Could it be a simple as losing the old 2 blade prop on the Mercury 9.8? Maybe a three blade would prop would help?
What really got my attention was how much wake the boat makes, it makes more wake than my 19' boat with a 115hp Evinrude on it. When running wide open, the stern of the boat and rear of the gunwales are below the surrounding water, even with nothing but me and the motor onboard?
It's like it's still pushing the boat downward not forward. I can't bring the motor in any further and moving it outward makes the bow really rise up.
To be honest, I was pretty surprised at how that little 9.8hp motor can make the bow rise up, lifting a 250lb+ passenger so high in the air. If it was just a matter of not getting on plane but pushing water, I'd just have figured the motor was too small but it seems like it needs to be tilted in even further???

 

[marcortez]

Re: Boat won't plane out?

Ditch that 2 blade prop and put on a 3 blade.

I am confident you will be pleasantly surprised by the new found thrust.

 

[ovrrdrive]

Re: Boat won't plane out?

I'd try a 3 blade prop and possibly a set of smart tabs to get a little more stern lift.

 

[crackedglass]

Re: Boat won't plane out?

Ditch that 2 blade prop and put on a 3 blade.

I am confident you will be pleasantly surprised by the new found thrust.

Any idea what prop to try first? pitch and diameter?

I can't find anything online as far as recommendations for a 9.8hp Merc at all.
Its an exhaust through prop motor. I'd guess somewhere around a 10.5 x9P or so? The dealer here wants $240 for a new prop and can't tell me whether its a 2 or 3 blade prop by the part number, so their out. I need to know what I'm ordering and if it'll help or not.

 

[amclaker]

Re: Boat won't plane out?

You've probably all ready checked this but on some of the mercs they had a real tricky shallow water drive mechanism. Are you sure the motor is going all the way down?

 

[tschmidty]

Re: Boat won't plane out?

Iboats should be able to provide a prop for you. What year is the motor? And yes a 3 blade will do much better than a 2 blade. But I think you are going to be in tough straits. You can try dropping the motor down a bit. But the problem is most likely your weight at the back of the hull. I am surprised a tiller extensions didn't help much. Smart tabs would probably help too.

 

[crackedglass]

Re: Boat won't plane out?

Iboats shows nothing for the 1973 9.8hp Mercury.
The dealer only shows the original prop, which they say is obsolete and not replaced.
So far no luck finding a 3 blade for a 1973 9.8 Merc.

I moved the tank permanently to the bow, I found a v shaped bow tank that holds 6.4 gallons and made a bow deck with a filler in it. It adds about 50lbs to the bow while removing about the same from the stern. I left the battery at the stern, I couldn't see adding 10' of heavy copper cable to run a rear trolling motor. The tank up front didn't do much, it's better but not right. The only fix would be me moving to past the front half point of the boat. That's not possible. The motor is in as far it can be, the pin is in the last hole and the motor is latched on the pin. Even without the pin, it won't go any further inward.
The impression I get is that the motor is more removing water from under the boat then moving it forward. Thus the huge swell or wake it creates. The boat sinks nearly 18" on a full throttle take off. While at full speed, even with one person, (ME), sitting on the middle bench, with a tiller extension, the water adjacent to the stern is higher than the top of the transom. It's as if the motor is still pushing the boat down not forward even though the motor is at a negative angle to the boat. I've even tried shimming it a bit, as much as a half inch to see what happens.

I hung a buddies 20hp Mercury today, just to see how it responds, but it did the same thing, it still dug in and plowed. The boat goes faster but don't really feel like its on plane. more than 80'5 of the hull is still in contact with the water, the bow rides high and the top speed is only 13 mph on the GPS. That same motor moves my 16' Mirror craft aluminum boat over 25mph with two big guys in it. With the boat being completely on plane. I've watched my buddy do a few laps around the dock with it and the Mirro Craft hull is barely in the water. Maybe the last few feet are in contact with the water with the Mercury 20hp. The 14' Starcraft, a much lighter boat plows and digs in at full throttle. The Mirro Craft 16 is about a foot wider and 1.8' longer.

One very noticeable difference between the two hulls is that the Starcraft Sea Farer hull has no liner ribs on the bottom, just a keel that runs partway back. The Mirror Craft has four strakes riveted to the bottom and a full length keel. Of course, I'd think that these would add directional stability not buoyancy. At rest, the Starcraft 14 has about 9" of freeboard at the stern. More than what I'd consider a comfortable margin. On a fast take off, the Mirro Craft jumps up and takes off, the Starcraft sinks down and moves forward. Added weight has very little effect on the Mirro Craft, even with three guys on board, with the two heaviest at the stern, the Mirro Craft will still plane out with the very same Mercury 20hp, and still out run the Starcraft even with the 9.8hp motor on it. Even a buddies 15' Grumman does better with far more weight and a 9.8 Mercury, we even swapped motors just to see the result.
I took the Starcraft and had it weighed this morning, I took an old refrigerator box, flattened it out, took it to a buddies junk yard scale, took the boat off the trailer, motor tilted up, battery and full tank of fuel in the boat, two paddels, two tackle boxes, and no passengers and got a weight.
The boat alone weighed in at 417lbs. Me and a buddy got on the scale, the total was 561lbs, which would be the weight of the boat in the water.

The 1967 Brochure lists the Sea Scamp at 700lbs max persons and gear. We're way under the max weight, the boat is rated at a max hp of 20 on the CG plate. (I don't have a catalog for the 1973 but am assuming that the 1967 is similar). Either way, we're well within the weight limit of the hull, and with the tank now forward, the balance with the 50lb tank, and one 250lb person at the front of the boat, it would wll offset my weight and the 80lb battery at the stern running the tiller motor. More so, with me riding on the middle bench, with a tiller extension should even more so balance out the boat. I'm leaning more and more towards it being a prop issue. I feel if the prop was able to grab enough water to get the boat moving, it would get up on plane an stay there.
The 20hp motor I was using also runs a 2 blade prop, which works very well for maximum top speed on the other boat.

 

[cr2k]

Re: Boat won't plane out?

Just do what everyone else does...Get an earlier John/Rue 15 and a set of 9.9 decals.

 

[crackedglass]

Re: Boat won't plane out?

Just do what everyone else does...Get an earlier John/Rue 15 and a set of 9.9 decals.

That don't work here, they check model numbers. If the model number is removed, they look further as if it's stolen.
Besides, the 9.8 Merc is paid for and has super low hours. It came with the boat which was last registered before I got it back in 1975. It and the boat look new.

So far I've looked everywhere and can't find a 3 blade prop for a Merc 110. Only a two blade prop. Apparently there were also two different spline counts for that motor over the years as well.
As much as I hate to do it, I may try a hydrofoil on the motor, it may be the only answer that won't really drag down the boat's speed. smart tabs on such a small boat with such a small motor will no doubt be a huge drag.

I had it out today and what appears to be happening is that the prop is essentially evacuating the water from beneath the boat but isn't able to fight the wind and tide. The stern is below the surrounding water and the prop putting out a wake comparable to a 8500lb boat. While underway, there's two huge fans of water coming from both sides of the prop, probably a good 20" tall on each side, then the wake formed by the hull itself plowing through the water. There was a 5 mph breeze blowing head on, and the water was moving at a 45 degree angle to my direction. It took nearly 45 minutes to cross about a square acre of lake water. This was with just me, sitting in the middle seat, no battery, no trolling motor, and a 3 gallon fuel tank in the bow. The motor certainly is running strong, if not, it wouldn't be pushing so much water out the back. More than once I looked back and could see the wall of water on three sides following the boat. My old 26' Owens didn't make a wake that big.

Even when I set the throttle and move forward, in an empty boat, it still acts the same. I'm pretty convinced I need to find a 3 blade prop, and there's no more weight I could move forward. In fact, I can hear the prop free spin or cavitate on occasion with my weight up front and none at the stern. Being a basically a smooth flat bottom boat, that's hard to do.
The motor feels strong, it could also probably turn a larger prop or one with more bite, but I feel diameter may also be an issue. If I got to Mercury, they list a 9"x9P and a 9"x10P, in two blade, and a 9"x7P in a three blade, which is listed as a low speed, work boat prop.
This doesn't coincide with the prop that's on it, the current 2 blade is not 9" in diameter, it measures only 7 3/8" tip to tip. There are no markings on the prop anywhere. So after seeing the OEM prop chart, I'm not only thinking I need a three blade prop, but that I may have an extra small prop on this motor? A larger prop would fit, maybe even a 10" would fit. The two blade does look tiny to me on this motor but its my guess that it's the prop it left the dealer with when new, and I have a second well worn spare that came with the boat that's the same size.