9.9hp vs 15hp on displacement hull

Dave Diefenderfer

Dave Diefenderfer

Well-known member
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As many have followed, I have a heavy (375lbs) 12ft x 5ft displacement hull sneakbox. My 15hp Johnson is not reliable enough for me, and I am in the market for a new OB. Having spent too much on the last motor and repairs, and still not satisfied, I am likely going new. So now the issue with the 4 stroke is the weight. With my 15hp, I could get 13mph per the GPS, but loaded, really more like 9-10mph, which is fine, I am not going that far. I stopped in BassPro, they had a 15hp Merc for $2300, and a 9.9hp for $2000. The difference in weight is 115lbs vs 84lbs. Has anyone run both a 15hp and a 9.9hp on the same displacement hull? How significant was the difference? I don't want to put a 9.9hp on and then run it at full throttle all the time. But I the upsize to the 15hp in both weight and bulk is significant. Having them sitting side by side on the rack, that 15hp is big!

Input?

TIA, Dave
 
Dave, the experience I've had with the Holland Point sneak boat is that past a given HP (6 in this case) it doesn't make any difference how much more hp you have. Personaly I would go for the lighter weight on a smaller boat.
 
Rule of thumb displacement hull speed in nautical miles per hour is square root of length at water x 1.3.

To push a 12' displacement hull > 7-8 mph you must be pushing a heck of a bow wave.

I agree on 2 stroke. Much lighter.

From the choices you offer buy the 9.9 and save 30 pounds.
 
Rule of thumb displacement hull speed in nautical miles per hour is square root of length at water x 1.3.

To push a 12' displacement hull > 7-8 mph you must be pushing a heck of a bow wave.

I agree on 2 stroke. Much lighter.

From the choices you offer buy the 9.9 and save 30 pounds.
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Yes, as you suggest he is exceeding hull speed by quite a bit and the hull is planing. My only concern is if the 9.9 will get him what he wants given the large amount of ponies to operate under those conditions.

T
 
Rule of thumb displacement hull speed in nautical miles per hour is square root of length at water x 1.3.

To push a 12' displacement hull > 7-8 mph you must be pushing a heck of a bow wave.

I agree on 2 stroke. Much lighter.

From the choices you offer buy the 9.9 and save 30 pounds. So, I should only be running this hull less than 5mph? (~10ft waterline?, sqr=3.16X1.3= 4.1knots = 4.7mph)? I suspect the 9.9hp will easily do that, and not be running at full throttle to do it either. Is this math correct? I always thought a boat was planning when the effort to maintain speed drops off? Always learn for this site! Thanks guys. Dave Sorry, typo.
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[font=Verdana,Arial,Helvetica]"I should only be running this hull less than 6.7 mph?[/font]"

Yes, if you want max efficiency and the best handling.

Hull speed is the theoretical speed that a hull can travel without planing (max displacement speed). Displacement hulls are designed to move their mass very efficiently with small engines and are much more efficient than planing hulls when operated at displacement speeds. They can go faster than hull speed, but they become much less efficient than planing hulls. For example, I expect to get 20+ MPH out of my 15+' planing hull with a 9.8 HP MercHatSan at full power, but I'll probably get 6-7 MPG if I am lucky. It will handle well on plane and handle poorly at displacement speed. You will probably get ~10 MPH with the same motor on your hull at full power, it will get 4 MPG and handle like a garbage scow. However, you can can run at 6 MPH at only ~1/4 power of your available power, get 12+ MPG and the hull will behave beautifully. (My numbers are ballpark estimates to illustrate the concept; real world numbers will differ, but the basic concept will hold true.)

Your hull speed is around 6.7 mph by your calculation. I'll bet money a 5 HP motor will get you that speed even with a full load! Of course the first time you have to fight upstream against a hard falling tide that is being pushed out to sea by a nasty north wind, you will cuss me as you stand still under full power. Due to that, I'd use the 9.9 HP. That should give you enough reserve to get out of the sort of trouble that duck hunters get into, yet still be fairly light. If you keep your speed around 6 MPH the majority of the time, the motor should sip gas. Stow fuel and batteries forward to balance the heavier motor on the transom.

[font=Verdana,Arial,Helvetica]"I always thought a boat was planing when the effort to maintain speed drops off."

You are absolutely correct... but a planing hull is much more efficient getting to that point. Let's say a planing hull can get just get on plane at 12 MPH with a 6 HP motor at full throttle. A equivalent displacement hull with the same size motor will wallow along and never plane at full throttle. It will take a 15-20 HP motor to get the hull on plane. The hull will only plane at a higher speed.
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Nate
 
Rule of thumb displacement hull speed in nautical miles per hour is square root of length at water x 1.3.

To push a 12' displacement hull > 7-8 mph you must be pushing a heck of a bow wave.

I agree on 2 stroke. Much lighter.

From the choices you offer buy the 9.9 and save 30 pounds. So, I should only be running this hull less than 5mph? (~10ft waterline?, sqr=3.16X1.3= 4.1knots = 4.7mph)? I suspect the 9.9hp will easily do that, and not be running at full throttle to do it either. Is this math correct? I always thought a boat was planning when the effort to maintain speed drops off? Always learn for this site! Thanks guys. Dave Sorry, typo.
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Nate has a lot of good stuff in his reply, I just have a couple things to add. From seeing the speed that people run these boats, it is very common for people to run these displacement hull sneakboxes above hull speed. If I had one I would, no question.

As Nate said, if you want max efficency, you should stay below hull speed and could probably push the boat with a 2 hp to hull speed. If you want to exceed hull speed it takes a lot more hp to push the hull. When you push a hull above hull speed, much of the hull comes out fo the water and you have less wetted surface and the surface that is wetted is less of an arc (more flat) and more likely to plane than the whole hull. You need a lot of hp to get the hull up on plane. Above hull speed you aren't operating under ideal conditions as far as hull design, but obviously good enough since "everyone" (or at least a lot fo people) do it. My only concern and the point I was trying to make is that I'd be concerned that the 9.9 may not make it completely over the hump (as in Nate's example at the end of the post). Without knowing your boat exactly, the 9.9 may only allow you to get to hull speed (at 25% of hp) and the extra 75% of hp not push you over the hump, so you will have a 5 mph boat, not just a little bit less than what you saw with the 15. There is a big differance between 10 an 15. I don't know what to say, you need to try the 9.9 to see of you are happy. I woudl be really unhappy with 5mph if you are used to 10, plus if the 9.9 isn't big enough you are getting the same performance out of a heavy and expensive motor that you coudl get out of a 2-3-4 hp motor that is lighter and cheaper.
 
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Just to interject - this was a very informative thread for me - thanks Nate and Tod, you cleared up a few things I'd been wondering about. Good luck motor-hunting Dave!
 
This is great, why I love this forum.... turns out a co-worker has a 9.9 evinrude. The same motor, just de-tuned from the 15hp Johnson I had on the boat, I will borrow it, hopefully this weekend and give it a try. This will answer the HP issue. I am thinking I should bring some weight too to simulate the extra 35lbs on the stern that the 15hp 4 stroke might be... it won't be with a 15hp, but perhaps I can extrapolate from what I see and the GPS readings?

I'll keep you all posted.

Dave
 
Not to beat a dead horse but go with a 15 HP 2 stroke for a couple reasons.....the weight # 1 but secondly the 15 HP 4 stroke has a large profile when you are trying to hide the boat.

If you came here to Jersey in mid season where the BBSB is in every marsh in the state 95% of the guys run a 2 stroke for the reasons I mentioned.

Best of luck with your choice.
 
Brian, I would buy a 2 stroke new if I can find one. I agree, the 15hp 4 stroke was huge, not sure I can shoot over and around it on a sneakbox, but I am snake bit on spending big bucks on a used outboard. If I found one from someone I knew, or can find a new one left over, I will do that for sure. This thread is the what if/what's next ....

Dave
 
Another thing you might want to consider, and it depends on how your sneakbox's hull is designed, but when it gets rough I like to have the extra torque that the 15 gives me over a 9.9. I can get my bow up into the waves and push through when the boat is loaded up with decoys.

Now that is how my boat rides, and each hull is different, just something else you might want to take into consideration.
 
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