Since you asked- 9.9 versus 15 kicker

M

Matt Gurnsey

New member
So- if 9.9 is good, 15 must be better, right? Since up until recently most 9.9's and 15's were built on the same powerhead there was no weight penalty for going bigger. And if Smiling Bob is to be beleived, bigger is always better.

Well, maybe.

First we have to understand "hull speed". Hull speed is the teoretical maximum speed a boat can go through the water at dispacement speed.

Here's the wiki page: http://en.wikipedia.org/wiki/Hull_speed

Basically, hull speed will usually be 1.34 to 1.5 times the square root of the length of the waterline.

If we assume a 22 C-Dory has a 20' waterline, then it's displacement speed is roughly 6 knots. To acheive 6 knots doesn't take a lot of horsepower. Look at some of the 40' plus trawlers running 120 Lehman deisels to acheive 6-7 knot speed.

To acheive a faster speed requires a unique hull design, or a LOT more power to get the boat to plane. (Hull design is a whole other long topic!)

So a 6 horse motor will push a 22 C-Dory to hull speed. And 8 or 9.9 will give extra torque, and some reserve for wind.

Here's where some guys get a bit confused. Often a customer will say he wants a 15 horsepower kicker to overcome tide should he be stuck offshore with the main down. It's a long ride home at 6 knots.

Here's the thing- to paraphrase Mr. Scott- "You canna change the laws of physics". So if your boat has a 6 knot hull speed, and you are going against a 6 knot current, you will basically be standing still. Unless you have enough power to get the boat on plane, and your kicker isn't going to do that.

I had a boat owner call wanting pricing on Evinrude's 60 horsepower motor. After some discssion it turns out he has a 28 Skipjack with a 7.4L Main engine, and he wants to build a custom bracket and use the 60 for a kicker motor so he can get back at better thanh six knots should the main go down. He's talking about spending $12-$15K on this project. After much discussion I talked him out of it. 60 is not always better than 10.

Another example- We had a boat in for service- a 24 Sea Sport or Osprey (I don't remember exactly, but that type of hull shape). On it was a custom built kicker bracket with a Yamaha F25 four stroke kicker motor. Because if 15 is better, than 25 must be great!

So with close to $10K into this set up does the owner get more speed? One look at the prop tells the story. The paint on the edge of all the blades is worn away. When the owner advances the throttle, the kicker motor provides more power until the boat hits hull speed. Once there, additional throttle increases power output, but not enough to get the boat on plane. Something has to give, and the prop starts to cavitate, and the exploding air bubbles wear away the paint.

This owner has spent a lot more money, burns more fuel, and maybe gets an extra 1/4 to 1/2 knot out of the kicker motor.

He would have been better served with a Yamaha High Thrust 9.9. Only ten horsepower, but a slower turning, larger prop with better bite in the water. Like the "granny" gear in a truck, it gets the heavy load moving at slower speeds.

So (to answer the question that started all this) what should one have on a 25 C-Dory? Theroretical hull speed is about 6.5 knots. I would suggest any of the high thrust 9.9 products on the market as being a better match for kicker duty. Merc does make a 15 kicker, and if you must have 15 then it might be an option.

As a commission sales person I make more off of the 15, but the 9.9 will do everything you need for less. I would rather sell you the right tool for the job then more than you need.

Now, I have to go join Mr. Scott for further physics lessens, and the tasting of a liquid he describes as "green".
 
All of that physics was nice and helpful and all but the real issue comes down to digits. 90 and 9.9 just go well together. 90 and 15, not so much.
 
So, Dr. Matt, how do you compute the hull speed of a cat? What size kicker, mounted between the mains, would push a TC-255 to hull speed? Not so much for trolling but for economical low speed exploring.
 
Yes, and 150 and 15, 150 and 9.9 not so much. Would like to hear from Les, he hung the 15 on Daydream, as I recall he said either would work but we might like the 15 better for the extra torque, which if I read Matt right is pretty much non-existent.

Capital Sea":ut7ybd3l said:
All of that physics was nice and helpful and all but the real issue comes down to digits. 90 and 9.9 just go well together. 90 and 15, not so much.
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That was interesting reading. How do I figure out the size of motor for my 19' Angler? What do I need to overcome strong wind and rough water on Yukon Lakes? And where should it be mounted? On the transom or on a bracket?
Thanks, Peter.
 
I understand the hull speed issue. However when I find myself in the bottom of a trough I'll take the extra horsepower to push the stern around and straighten up the boat before the next wave arrives.
 
Matt Gurnsey":19c2eyh4 said:
So- if 9.9 is good, 15 must be better, right?
<<Stuff Clipped>>

As a commission sales person I make more off of the 15, but the 9.9 will do everything you need for less. I would rather sell you the right tool for the job then more than you need.

<<More Stuff Clipped>>
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I sure wish that this was the philosophy of every person selling any product...

Hats off to Matt!
 
Interesting timing. I just had Les install a Tohatsu 9.9 on the 25. I'm looking at merely a get home/get to a safe harbor kicker, not a trolling motor. It is not a hi-thrust model, as those come only w/ power trim/tilt and I wanted the KISS principle. He advised it should do fine w/ one caveat; depending on tide/wind conditions. I asked about the 4 blade (hi-thrust) prop and he said to try the boat w/ the standard three blade first. He said I'll only get power up to about 3/4 throttle anyway; after that there won't be any discernible difference.

Back when I bought the boat from Matt I inquired about kickers. He told me at that time that a 9.9 was more than sufficient. It's good to see he still thinks so! :wink: And Matt, I commend your honesty and putting the customer first. :thup Not all salesmen have your integrity.
 
Dreamer":2a0jezlk said:
So, Dr. Matt, how do you compute the hull speed of a cat? What size kicker, mounted between the mains, would push a TC-255 to hull speed? Not so much for trolling but for economical low speed exploring.
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Well, you had to go there, didn't you! :lol:

Cats are a whole other animal, so to speak. Long skinny hulls are easier to push through the water, and can exceed the 1.34 to 1.5 rule of thumb for hull speed. Look at how fast a kayak can move through the water- based purely on waterline lenght they should not be able to go as fast as they do, and they certainly don't plane.

Glacier Bay is another example. Their hull is a displacment hull that can be driven to a much higher speed that the hull speed of a similar size mono hull. It doesn't plane.

The Tomcat uses a planing hull design, but at displacments speed have the ability to go faster than the same size mono hull.

That all being said, I think a high thrust style 9.9 will have better perfromance than a standard 15 because of the gear ratio / prop style advantage. Additionally, you'll probably need an extra long shaft length, which may only be available on the 9.9 models.

But I don't have any first hand knowledge on kickers on Tomcats. I know we got asked a lot of questions about them at shows. My guess is that the 9.9 that will push the 25 Cruiser to 6.5 knots might push a Tomcat faster because of the hull shape, and be able to use that top 25% Mark was talking about..
 
Pat Anderson":2rg7i6xs said:
Yes, and 150 and 15, 150 and 9.9 not so much. Would like to hear from Les, he hung the 15 on Daydream, as I recall he said either would work but we might like the 15 better for the extra torque, which if I read Matt right is pretty much non-existent.
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The 15 will have more torque if it's a bigger displacement, which all of the brands now are. Of course this comes with a weight penalty, which the 25 can handle better than a 22 could.
 
What about the head wrenching, neck snapping acceleration the 15 provides over the 9.9 ? How come we don't talk about that ?
I wanna see some zero to hull speed numbers.......
Marc
 
This is what I have been saying for many years. We have pushed a 65,000 lb boat with a LWL of about 50 feet at 2.5 knots with a 4.5 hp outboard on a dinghy. I grew up on a 26' sailboat which had a hull not unlike a C Dory 25 (Arc bottom, fin keel) which could plane/surf under certain circumstances. Initially we had a 1932 5 hp outboard, and it pushed the boat about 5.5 knots. That motor finally bit the dust, and my dad went with a "modern" 9.5 hp--there was no increase in speed. Most likely a 6 hp is plenty for the C Dory 25, and I have used a 3.5 hp dinghy motor on both the 25 and 22.

The C Dory Tom Cat is a bit different. As I recollect the Black Feather 30 boats use am 8 hp high thrust outboard in the center for trolling. (These are 30 foot "Tom Cats", now out of business). However, the standard 255 Tom Cat has a disadvantage. It is not a cat at displacement speeds, because the tunnel is not high enough, and there is drag of the wing deck aft which makes it more like a barge. There are some cats which will perform very well with small outboards. For instance I know of a 32 foot wave piercing cat which is a semi displacement boat, and weight of only about 1500 lbs total. This boat will do 12 knots with a 15 hp outboard.

As far as props on kickers, you want the lowest pitch you can get, and the largest diameter--this is what the "high thrust" motors allow. A larger prop, and lower gear ratio--the engine is the same.
 
Is there a gearing difference in the "hi-thrust" models or is the lower unit the same [gearing] and the only difference the prop size/pitch?
 
Usually in the small motors its just a prop. In the 50/60 HP range its a lower unit of the next size up gear case as found on a 90/115 HP to turn a bigger diameter wheel .Usually found on pontoons and work boats .
Marc
 
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