Limited to 9.9hp

[slowleak]

I live in an area with a limit of 9.9hp motors on most lakes.
My boat is a 1967 Starcraft 14' Sea Scamp. My current motor is a very clean same year Mercury 9.8hp two stroke outboard.
I have two issues with this set up, first off, the 9.8hp will not plane the boat, second, on take off and while underway, the rear gunwales are beneath the level of the surrounding water. I've borrowed other same size motors and got the same results. I've tried all four available prop options for this motor, the best overall is the original 2 blade prop it came with. Three blade props all seem to blow out under load, the two blade prop never loses its grip and gives the best top speed, which is about 7.2 mph on the gps in the 14' boat. I get maybe 6 mph with the same motor in the 16' boat.
I've also tried one 15hp Johnson motor which other than having a bit more top speed, it still didn't plane the boat.
Weight wise, the boat is empty other than a single 3 gallon fuel tank which sits just ahead of the rear bench seat, two clamp on plastic seats, and myself and a fishing buddy. I weigh roughly 320lbs, my buddy is maybe 300lbs.
I've stayed with the 14' boat since a 16' boat would further increase these issues. The 14' boat is rated at 700lbs tops, the 16' at 800lbs.

I did try a 20hp motor once but while faster, it would not lift the boat up on plane.
The 20hp was faster but it sunk lower in the water under power, again this was regardless of the trim pin position.
I also own a 16' Starcraft Sea Scamp, but performance with the maximum 9.9hp motor make it all but useless on a large lake with a strong current in the main channel. It does great with a 40hp outboard but I can only run that motor in tidal saltwater here. The 20hp motor made a huge wake, lots of spray but no speed and the boat never got fully up on plane. Moving the trim pin one hole either way made the difference of plowing water up front or riding bow high with even less speed.
Neither boat is heavy, I can move either boat on and off the trailer by hand myself so I don't suppose either hull is more than 300lbs or so. There is no hidden or water weight since both are open hulls with wooden bench seats.

The added drag of the larger hull along with the way undersized motor make moving around the lake under power not much better than rowing by hand.

I've been thinking about a new motor too and am wondering if the newer 4 stroke motors will make this situation worse? With a heavier motor, it will further hinder any chances of getting the boat up on plane.

Is there a fix for the rear of the boat dipping so low in the water under power? (It does the same thing with a minimum load and a 110lb person at the tiller, regardless of trim position). Its as if the motor is pumping all the water out from under the stern faster than the boat can be pushed on top of the approaching water.

 

[jbjennings]

Re: Limited to 9.9hp

620 lbs. of stout men is a lot for a 9.8hp to plane. I'm thinking this boat is aluminum, right? If so, does it have a floor? If there is water under the floor...........

A 300lb. hull (that's pretty heavy for a 9.8hp) with a 150lb person at the tiller might plane with a 9.8hp, but not a lot of power to spare. No chance with 2 big guys.
Can you post a pic of your rig? If your transom is made for a long shaft (20") and your motor is a short shaft (15"), that's one problem. I'm thinking based on your comment that the gunwales are below the water when underway, that you most assuredly have FAR too much weight. Either there is water soaked in to the floatation foam under the seats or floors, the boat is just plain too heavy, or you boys need a bigger boat.
JMO,
JBJ

 

[Scott Danforth]

Re: Limited to 9.9hp

you need a bigger boat.

the your 320#, your buds 300#, the motors 65#, the fuel tank's 20#, and the two seats and your fishing gear is upwards of 800#

over your 700# limit.

I have been on 14' boats with the 9.8 and they almost have enough to plane if you are in the right direction with wind and waves. the 9.8 will push my wooster hellion at 25mph, however that has a 300# weight limit.

 

[JimS123]

Re: Limited to 9.9hp

My 14' tinny weights 175 pounds. With 550 pounds of people, a 6 gal tank and assorted cooler, tackle box etc. she planes very nicely with an older 9.5 'Rude with the std 3-blade prop.

Your boat is waterlogged big time.

 

[slowleak]

Re: Limited to 9.9hp

There's nothing in the boat to be waterlogged, its all aluminum with three wood seat planks. No flotation, no floor. The factory manual that came with the boat when new lists the hull as weighing in at 150 lbs but hanging on a hook, the bare hull is 293.4 lbs.
It has two chunks of foam flotation under the middle and rear seats, both foam blocks are dry and weigh less than 5 lbs combined.
The fuel tank weighs 32lbs when full with the fuel line. The motor weighs in at 67 lbs with a two blade prop full of oil. The two seats, swivels, and clamps weigh 26 lbs. I also normally carry a small battery pack to power the nav lights if I get caught out after dark. That adds about 12 lbs.
The 16' boat weighs in at 440lbs bare, its got aluminum seats with foam inside the very front seat and rear seat. The foam is dry and easily removed. There is no floor. The 16' is 10" wider.
Both have 15" transoms, all motors are short shaft or 15" length.
One thing I should point out is that the ratings on these hulls has changed over the years, the OEM rating was low, if I go by the Coast Guard ratings for a hull of this size, I'm well withing the limits. I also noticed that newer models, in the very same hull are rated much higher with no changes to the actual boat or design.
The boat don't ride low in the water and even with a single very light man in the boat the same problems persist.
So far I've tried the following motors, my 1974 Merc 9.8hp, my 20hp Merc 200 20hp from 1970, a 1985 9.9hp Evinrude, a 1987 15hp Johnson, and a newer 9.9hp Tohatsu but all act the same way.

I used to run the 9.8 Merc on an older Sears boat and it ran much faster but the boat was unstable with just myself in the boat, my weight alone would nearly sink the boat.

Set up with the legal 9.8hp Merc, when I give it gas the boat sinks into a hole in the water and moves slowly forward. From that point on it remains that low in the water. If I bail out of the throttle, the following wake comes dangerously close to swamping the boat from behind. The boat however has always popped right back up to normal height the minute the motor stops.
The real problem at hand is that the lake has a 9.8hp limit and a larger boat would have issues getting back to where the fishing is good. The 14' has enough issues. I tried a jon boat but they don't have enough freeboard or capacity, and the motor had issues moving the boat at all in strong current.
A buddy runs the same boat but with a few year newer motor added a hydrofoil, he doubled his mileage, is able to get on plane but lost top speed. I've also tried a tiller extension and driving from the middle of the boat but when I do that the prop sounds like its out of the water as soon as it gets moving, and the hull still rides in a hole so to speak.

Two guys can carry the boat, motor, and all contents, I've fished a few places where there's no ramp and have lifted the whole rig over a guard rail and to the water.

With any motor, I've always made sure the anti-ventilation plate is slightly above the bottom of the boat.
What surprises me is how much water is being moved behind the boat, my 30' cruiser never made a wake that big.
If the motor wasn't just clamped on, I'd consider trying a hydrofoil but I'd be affaid that 'lifting' the hull that way would work the motor up the transom. The motor has less than 20 hours of use since new, it was completely gone through just last summer, it spent years in storage after the original owner mothballed it and the boat back in the mid 70's. Both the boat and motor look brand new. The 16' boat is newer, but since it had no hull numbers its been titled as homemade as a 2013 model. It appears to be a late 70's model. Both boats are stored indoors and are dry.

I don't argue that I'm heavy for a small boat but the lake and the local regs don't give me much choice. I can't very well take a 20' boat into an 8' wide feed creek or run it in the lake with either a trolling motor or 9.9hp motor.

 

[steelespike]

Re: Limited to 9.9hp

Makes no sense.You do take it off the trailer? lol
Have you tried the 9.8 on another boat with the 150 pounder at the tiller?
Are we sure all the motors are sound.good compression.All cylinders working?
Have you tried pulling one plug wire at a time and see if it will run?
You say its supposed to be 150 lbs but registers 293 in the scale. Then you say 2 guys can carry the whole rig.???

 

[slowleak]

Re: Limited to 9.9hp

Makes no sense.You do take it off the trailer? lol
Have you tried the 9.8 on another boat with the 150 pounder at the tiller?
Are we sure all the motors are sound.good compression.All cylinders working?
Have you tried pulling one plug wire at a time and see if it will run?
You say its supposed to be 150 lbs but registers 293 in the scale. Then you say 2 guys can carry the whole rig.???

Me and my buddy can carry the rig with the motor off the trailer and into the water. Two of the lakes do not have ramps or access, the only way to get a boat in the water is to lift it over the rail. A few of those lakes are electric motor only lakes and in those cases I run a rear trolling motor and a battery, in which the combo is actually heavier than the gas motor.
I figure that while lifting the boat, one of us is lifting about 150lbs, the other about 220 or so.
The same 9.8hp motor makes a 12' boat fly with a smaller driver, but with me in the 12' boat it rides too close to taking water over the bilge.
The 14' Starcraft was weighed on a meat scale when I first got it, we hung it from the bow eye to weigh it at work.
Since the same scale weighed the motor and other items at or near their listed weights I believe the weight is correct.
It weighed my Alumacraft 14' boat, which I just sold at 111 lbs total, but that boat had no freeboard and only a 340lb max weight.
The 700 and 800lb weight rating states maximum persons 4 or 700lbs on the 14' boat, persons, motor and gear is 940lbs.

My 9.8hp has 140 psi compression on both cylinders, its running 100%. Its basically a new motor that sat for a long time in storage. I replaced the water pump, rebuilt the carb, and replaced the fuel hoses and it's been fine.
I think the issue has to do with the undersize motor and prop design. On the very light hull, it hops right up on plane and gets moving pretty well. On my Starcraft, it just sinks into a hole and plows.
The hull is perfect, no dents, no extra weight, nothing. Other than the clamp on seats its as it was when brand new, not so much as a scratch.
I thought about a bigger boat, but I'm still limited to the 9.8hp motor, and if I go to a hull over 16', I'll most likely need a longer motor. I've been looking at both a 1996 Sea Nymph modified V hull and an older 17' V hull both with just over a 1000lb max weight
The 17' boat weighs in at over 1000 lbs, and the Sea Nymph weighs over 1500lbs due to a console, decks, deck boxes, livewells, etc. Neither of those will work well with a 9.8hp motor.

Most guys run smaller motors and canoes, or run 12 or 14' flat bottom boats but most guys aren't 6'5" tall, 325 lbs.
I was also told that pulling one plug wire will damage the ignition on this motor? Both cylinders fire fine, the problem isn't making power. I went so far as buying a tach for the motor to be sure its achieving proper RPM, with the 2 blade prop I'm seeing 4780 RPM, with the best 3 blade prop it'll rev over 5500 RPM and on occasion burst above that. If the boat were 'heavy' with water, it wouldn't have so much freeboard and two of use couldn't so easily carry it.
The reason I bought this model boat is that years ago I owned the same model but with a Yamaha motor, that boat did over 25mph but I was 40lb lighter back then and that motor had a much larger selection of props available. I see very few options for the Mercury 9.8. All of my boats are aluminum.

 

[jbjennings]

Re: Limited to 9.9hp

The reason I bought this model boat is that years ago I owned the same model but with a Yamaha motor, that boat did over 25mph but I was 40lb lighter back then and that motor had a much larger selection of props available. I see very few options for the Mercury 9.8. All of my boats are aluminum.

If you weren't 6'5" and 325lbs, I'd say you were a lot lighter back then than 40lbs. when a Yamaha planed you out.
Seriously, everything you describe says too much weight. No chance of a pic of your rig?
JBJ

 

[JimS123]

Re: Limited to 9.9hp

Add it all up and there is no question that the boat is overloaded and unsafe to run under those conditions.

However, your first post said that it does the same thing with a light load and a 110# person at the tiller. THAT indicates a boat or motor problem or a combination of the 2.

 

[boobie]

Re: Limited to 9.9hp

Not to be in salting but I think there is a cargo weight problem here. Bigger boat, bigger lake would be the answer.

 

[tazrig]

Re: Limited to 9.9hp

About the best you can do is to put a 15 rude engine on it with a 9.9 cover on it and transfer as much weight upfront as you can. asking a 15 to plane on 1,000lb is still asking a lot.

 

[dkonrai]

Re: Limited to 9.9hp

or buy a johnnyrude 9.9 and put a 15 hp carb on it. there is some mercs that you can do the same, but i dont know which years.
dino

 

[slowleak]

Re: Limited to 9.9hp

Add it all up and there is no question that the boat is overloaded and unsafe to run under those conditions.

However, your first post said that it does the same thing with a light load and a 110# person at the tiller. THAT indicates a boat or motor problem or a combination of the 2.

The boat is not overloaded, it may well be close to max but its not over the limit. It makes it clear on the CG tag that the max persons weight is 700lbs, 300+325 is only 625. Max weight, persons, motor, and gear is 940lbs, so the total onboard is 625 + 69 + 32 = 726 then maybe another 30 or so pounds in fishing gear.

I tried something this morning down at the lake, if I leave the bow tied to the dock, give it throttle, it dips down in the stern just the same as when its turned free, if I reverse the motor, the rear of the boat rises up out of the water about 4" above normal freeboard. If I put the lowest pitch prop I've got on it, and give it gas super slowly, it don't dig in so low in the water. But as soon as I hit max RPM the boat starts to get lower in the water at the stern.
Its acting as if the prop is evacuating water from under the stern faster than the hull can advance atop it.

Test number two, I borrowed a buddies 7.5hp Evinrude with a hydrofoil on it, and gave that a try, and it took off just fine, but the clamps kept slipping on the transom eating up the wood. The 7.5hp had trouble getting around the middle knee brace on the transom, the clamp screws fall half on/half off the metal brace.
The little 7.5hp don't move very fast but it also don't make the stern sink super low in the water.
I think if the 9.8hp Merc had a hydrofoil it might actually get the boat up on plane, but I really don't like how the motor kept trying to slide up the transom. I suppose I could drill and bolt the motor but I hate to drill holes in a clean hull like this.

 

[jbjennings]

Re: Limited to 9.9hp

It will be interesting to see the results of the hydrofoil...... Maybe that would be just the trick.
JBJ

 

[JimS123]

Re: Limited to 9.9hp

The boat is not overloaded, it may well be close to max but its not over the limit. It makes it clear on the CG tag that the max persons weight is 700lbs, 300+325 is only 625. Max weight, persons, motor, and gear is 940lbs, so the total onboard is 625 + 69 + 32 = 726 then maybe another 30 or so pounds in fishing gear.

I tried something this morning down at the lake, if I leave the bow tied to the dock, give it throttle, it dips down in the stern just the same as when its turned free, if I reverse the motor, the rear of the boat rises up out of the water about 4" above normal freeboard. If I put the lowest pitch prop I've got on it, and give it gas super slowly, it don't dig in so low in the water. But as soon as I hit max RPM the boat starts to get lower in the water at the stern.
Its acting as if the prop is evacuating water from under the stern faster than the hull can advance atop it.

Test number two, I borrowed a buddies 7.5hp Evinrude with a hydrofoil on it, and gave that a try, and it took off just fine, but the clamps kept slipping on the transom eating up the wood. The 7.5hp had trouble getting around the middle knee brace on the transom, the clamp screws fall half on/half off the metal brace.
The little 7.5hp don't move very fast but it also don't make the stern sink super low in the water.
I think if the 9.8hp Merc had a hydrofoil it might actually get the boat up on plane, but I really don't like how the motor kept trying to slide up the transom. I suppose I could drill and bolt the motor but I hate to drill holes in a clean hull like this.

Just because its not over the USCG rated limit doesn't mean its not overloaded. Personally I would never run an outboard WITHOUT bolting it down. Once you drill the holes they get filled up with a bolt so there's really no hole any more.

Secondly, from personal experience with tiller tinnies (and with me and all the weight aft), I would never run one WITHOUT a DoelFin. (Forget the whaletails and other brands).

Finally, based on your first post and the one above, your own experiments point to a serious meachanical or design deficiency. Wish we could see pics of the hull and motor setup.


Hummmm....PS....just had a thought - could you have a straight transom? If so your tilt pin settings aren't working for ya.

 

[boobie]

Re: Limited to 9.9hp

I probably missed it but what's the boat rated for HP wise ??

 

[roscoe]

Re: Limited to 9.9hp

I believe the 67 14' sea scamp is rated for 20 hp.




That boat will probably never plane with that much weight in it.

It will never plane with that much weight, and only 9.9 hp. Just not going to happen.

 

[steelespike]

Re: Limited to 9.9hp

I think the size of the boat is holding you back.We had a 15 ft Lonestar bare tiller with a 25 Gale it wouldd struggle with 4 people and
gear. We replaced it with a 18 ft bare utility with the same 25 hp Gale. It popped on plane with 6 adults and gear.
The 9.8 might respond to a light 16 footer.
A 3 blade prop in the right pitch should do better than a 2 blade. The 2 blade lets it rev but can't push the load.

 

[slowleak]

Re: Limited to 9.9hp

The boat is fine, its a common 14' Starcraft tinny with a 9.8hp Merc.
The transom is about 10 degrees, give or take a few degrees I guess.
The motor has no holes where to bolt it up, but i suppose I could drill a few but they would fall below the wooden transom panel inside.
The fin on the Yamaha is a Stingray and its pretty wide.

I really don't see where I have many options here, a 16' or larger boat would be even slower and more underpowered and I don't gain much capacity over this boat, and legally I can't run bigger than a 9.9hp motor.
A larger boat will also present new issues getting back into the places where we fish.

When I first ran this combo, I thought the prop was bad, slipping or something, but after swapping in various new props I realize its not the prop. I would think that even a super heavy boat would still move along even though its not on plane, the fact that the boat sinks so low in the water is what gets me.

A buddy has a 24' boat with a small motor bracket on the back and a 6hp Evinrude, that motor moves that 2100lb boat better than this 9.8hp moves this boat.

If the 9.8 wasn't capable of moving water, it wouldn't be making such a huge wake behind the boat.

With both of us in the boat, there's about 14" of freeboard, the boat is pretty high sided, I'd venture it would take three times the normal weight to make it feel unsafe. With the normal load in the boat, the two spray rails down each side are barely in the water at the stern. There's about 6" of the transom below the water line at rest. On my 16' boat, there's far less in that the boat itself is not as deep and the bottom of the boat is more V shaped than on the 14' hull which is all but flat beyond about half way back.

I found a pic of the motor on the transom. In the water, the water line with us in the boat is just below the rear transom wood panel.
The motor is a museum piece condition wise, as is the hull.

 

[steelespike]

Re: Limited to 9.9hp

I think you my be better off with the OMC 9.9 IT has more displacement 13.2 comparerd to 10.9 of the merc.
Also; at least at least here at iboats; there are more props available 7" to 11" 3 blade props.
The OMC 9.9 also has a 2.42 gear ratio much better for heavy loads.
The 9.9 standard prop 9.5X10 is available in 6 distinct models including a 4 blade and a stainless.
The smaller less common sizes are also available in multiple models including 4 blade and stainless.
Larger diameter props do better with a load.
I note only a 9" X 8" is available here for the Merc.