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Author Topic:   16-footer: Maximum Speed with Low Power Without Planing
db4570 posted 10-15-2014 09:07 AM ET (US)   Profile for db4570   Send Email to db4570  
Hi everyone. I am an experienced boater, but just picked up my first Whaler, a 1971 16-footer (not sure of the exact model). I'm liking it a lot already.

It's in solid but ugly condition. All the wood was removed by the previous owner, so it's a big, wide-open hull, like a barge, which is exactly how I want to use it.

I realize this may be sacrilege to you guys who are really into your vintage Whalers, but bear with me. We would be using it mostly for a boat to putter around and haul supplies off the coast of Maine. We would be beaching it on a rocky shore, and while we'd make every effort to protect it, it will still pick up inevitable scratches and scrapes.

We don't need it to go fast, since we would only be traveling three to foure miles at a time, and not in any hurry.

I'm getting rid of the big old 85-HP engine on the back. I tested it out with my old 9.8-HP on the kicker bracket, and got about 6.5-MPH out of it. Not bad for this ancient motor, with the weight distribution all wrong, dragging the lower unit of the 85 along.

I want to get a new motor, and am looking at the 20-HP Tohatsu four-cycle engine. I'm wondering if the extra power will get me more speed, and at what point the hull speed maxes out. It would be nice if I could get 10- to 12-MPH out of it, and 15-MPH would be amazing.

Any experiences you can share with low power motors on Whalers would be helpful. Or if you can give me a rough idea of how fast they can go before going "over the hump" onto plane.



Jefecinco posted 10-15-2014 11:04 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
If I remember correctly hull speed in knots may be approximately computed by taking the square root of the waterline length of the hull times 1.34 but I'm sure a Goggle search would provide a better idea.

I believe five knots would be about the best you could hope for unless you want to plow with your transom half submerged. Your old 9.8 should be more than enough power to drive the hull at hull speed.


db4570 posted 10-15-2014 01:21 PM ET (US)     Profile for db4570  Send Email to db4570     
Thanks for the reply.

I have seen some of the formulas for calculating displacement speed, but they seem to be unpredictable at best. I think the unique shape of the Whaler hull might make it even more complicated.

So I'm looking for any real-world experiences. Like: How fast could you go on a small motor? At what speed does this size Whaler start to climb up on plane?


jimh posted 10-15-2014 05:14 PM ET (US)     Profile for jimh  Send Email to jimh     
The minimum horsepower to get a 16-footer onto plane with a light load is 35-HP, according to the data at

db4570 posted 10-15-2014 05:55 PM ET (US)     Profile for db4570  Send Email to db4570     
Thanks for the reply, Jim, but I DON'T want to plane the boat. I am trying to get an idea of the maximum displacement speed.
jimh posted 10-15-2014 08:36 PM ET (US)     Profile for jimh  Send Email to jimh     
db4570 first writes:

I'm wondering if the extra power will get me more speed...

db4570 later writes:

I DON'T want to plane the boat.

Thanks for clarifying your intentions. You want to increase speed by increasing power, but you do not want to increase to planing speed. Since 35-HP will reach planing speed, keep your power below 35-HP.

Let us say that for a Boston Whaler 16-foot hull with a light load the planing speed is going to be around 20-MPH. Now we have some data:

35-HP = 20-MPH (imputed from specification from Whaler for minimum HP)

10-HP = 6.5-MPH (from your data)

db4570 writes:

I want to get a new motor, and am looking at the 20-HP

Now we can project a speed for 20-HP. Based on the data above, and using just a simple linear projection, I will guess 11.9-MPH.

jimh posted 10-15-2014 08:47 PM ET (US)     Profile for jimh  Send Email to jimh     
db4570 writes

I am trying to get an idea of the maximum displacement speed.

The term displacement speed is generally understood to mean the speed when the length of the bow wave is equal to the waterline length. See

You can certainly push the boat above the displacement speed with more power. A speed greater than the displacement speed is obviously not the displacement speed, that is, the term displacement speed cannot mean two things at once.

What you seem to want to know is how fast the boat will go as a function of horsepower, except you impose an arbitrary upper bound that the boat must not be driven onto hydroplane.

db4570 posted 10-15-2014 11:30 PM ET (US)     Profile for db4570  Send Email to db4570     
I understand my goal is counter to what most people want to accomplish with a planing boat: to use it at displacement speed. This is mostly because I am trying to get out cheap and simple, with a small, light, tiller steer motor, using very little gas. Also, since it's going to be mostly a cargo carrier, and will often be loaded to capacity, I want to take it nice and slow, anyway.

I don't think a linear projection will work here, because the jump from displacement to planing speed is like a phase change. For instance, with my other boat, I can be at the maximum displacement speed, and I'll have to give it a big shot of throttle to get it over the hump onto plane. But then I can pull back on the power and it will stay on plane until power is reduced enough that it falls off. So it's not only not linear, it's not even a normal curve.

I understand that at a certain point at displacement speed, adding more power has very little additional effect on speed until it gets over the hump onto plane. So I'm trying to avoid that point of diminishing returns.

Basically, all theory aside, I want to get an idea what would be a comfortable displacement cruise speed, and what power would be required for that. Ideally, I'd try a 9.9, a 15, a 20, and see what works best. But then I know I'd get seduced by being right at that threshold of planing speed, and just another 10 hp would get me there...

I appreciate you guys helping me brainstorm this.


jimh posted 10-16-2014 08:06 AM ET (US)     Profile for jimh  Send Email to jimh     
Projecting with the power considered to the 0.5 exponent gives about 9.2-MPH.
frauke75 posted 10-16-2014 10:50 AM ET (US)     Profile for frauke75  Send Email to frauke75     
I had a similar question once regarding the same hull. (see link below)
I do not think you will get over 10 mph without planning.
I would say the hull will start to create a hump at around 5 to 8 mph. and you will need more motor to get over it. ...but then you have an empty hull. Above that and until on plane it is very inefficient motoring. (bow up and deep hole
Keeping weight forward, motor mounted high and maybe a fin might help a bit but the hull will eventually want to climb.
No science was used in the above statement... just my observation.

This thread will show you a lot of good scientific insight from members offering help with my quest for middle speeds.

jimh posted 10-16-2014 04:26 PM ET (US)     Profile for jimh  Send Email to jimh     
In general, one can say that all boat speed is obtained based on power in ratio to weight. In this case, where the Boston Whaler skiff hull is proposed to be used as a barge carrying a lot of cargo weight, consideration must be given to the total weight of the boat and its load in relation to the power available. This means that while a certain amount of power, say 20-HP, may move the hull at a certain speed when there is no cargo aboard, once the hull is loaded down with cargo and the total weight increased, the speed that will be available will decrease. Since the weight of little skiffs like a Boston Whaler 16-foot hull is so light, it is quite common that the increase in weight from crew and cargo becomes a very high fraction of the hull weight. This exaggerates the effect of weight.

One also must consider that most engines have a propeller whose pitch is selected with the notion that a certain speed range will be obtained. In the case of trying to plow along in a 16-foot skiff with a ton of cargo onboard, the propeller pitch should be changed to suit the anticipated speed range. If the propeller is not properly sized, the load on the engine will be too much, and the the engine will not be able to accelerate into its full-horsepower speed band. You won't get 20-HP out of a 20-HP outboard if you are lugging the engine along at 2,000-RPM instead of 5,000-RPM at full throttle.

Good luck with your project of making a cargo barge out of a Boston Whaler 16-foot skiff that was intended as a planing hull that can go 40-MPH. Let us know what results you get, if you don't mind, since you started this topic. It is always good to follow up. But I don't think that your use is really in the mainstream for most of us. Owners of 16-foot hulls usually have a 60-HP to 100-HP engine on the transom, and enjoy using the boat for its intended purposes.

tedious posted 10-16-2014 04:39 PM ET (US)     Profile for tedious  Send Email to tedious     
Skipping all the fancy theoretical stuff, in general you're not going to go a whole lot faster than that 6.5 you achieved. More than that and you'll be in "plowing" mode, where you're putting all the motor's energy into making a wake. You'll go a little bit faster, use a ton more gas, and your handling will be terrible - bow up, stern down, very difficult steering control.

My take would be to stick with the 9.8 , or at least try it on the center mount and see how it works for you, before investing more.

By the way, classic Whalers achieve their light weight by having skins like eggshells. If you're routinely beaching it on rocky Maine beaches you'll destroy it in short order - not just scrapes, but cracks in the glass, then water intrusion, and then it freezes over the winter and you're done.

I guess that would be another reason to not invest in a new outboard.


jimh posted 10-17-2014 02:07 PM ET (US)     Profile for jimh  Send Email to jimh     
There really is no "fancy theoretical stuff" in the discussion at this point.

The amount of power needed to propel a boat at a speed greater than the displacement speed (defined by the wavelength of the bow wave and the hull length) will be quite variable and depend on the hull design.

To get into a little bit of "fancy theoretical stuff" about it, I suggest reading the excellent book, PROPELLER HANDBOOK, by Dave Gerr. Mr. Gerr can offer a lot of advice on calculating boat speed in an understandable way, without getting very fancy or very theoretical.

Tim's mention of the fragility of the thin skin of a Boston Whaler boat hull is a very accurate observation. While not particularly relevant to the boat speed discussion, it is certainly a factor that ought to be considered. If one wants a barge that can be beached on rocky shores, I don't think a Boston Whaler 16-footer is the first choice that would come to mind.

Teak Oil posted 10-17-2014 06:32 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
The hull will plane at about 12-13 mph. What you want is a high thrust motor with a big round ear prop. I would suggest a high thrust 15, that way you can have good control in waves with a small motor. If you ever get caught in rough seas you will have no problem keeping the bow into the waves.
Also a high thrust prop will stop the boat on a dime with very little hp.

db4570 posted 10-18-2014 09:53 PM ET (US)     Profile for db4570  Send Email to db4570     
Hmm. Maybe I should have researched this a bit more before I bought. I assumed, incorrectly, it turns out, that Whalers were built like tanks, and this boat could handle some rough handling. But I should also clarify that we're not planning on abusing it, and not banging it around on the rocks. We would mostly be gently beaching it (albeit on rocks) to unload people and supplies, then mooring it. So it's not like it would be thrashed around on the rocks a lot.

I'm thinking about getting those keel guard type strips to protect it.

Anyone want to buy this thing from me, and I'll start over? It was a bit of a spontaneous purchase.

As far as the motor, I don't want to over-power it to try to get past its displacement speed, unless I really want to plane. There's a huge jump in cost, complexity, and weight between a 9.9 hp for displacement speed and a 40-50 hp for planing. But if a 15 or 20 hp could get my displacement speed up to maybe 10 mph, from about 6.5 mph with a 9.9 hp, that would be great. I just wish I knew if I'd gain anything going with that much more power.

I have to buy a new one anyway. I can't use the 9.9 I have now because 1) it's really old and 2) it's a short shaft and 3) it's really supposed to be for my rowboat.

This is an interesting discussion, and I appreciate all the advice.


jimh posted 10-19-2014 12:58 PM ET (US)     Profile for jimh  Send Email to jimh     
Most boats used where beaching on rough terrain is needed are built with metal, usually aluminum plates. You will find that in boating areas where there is rocky shoreline there are often local aluminum boat builders making suitable boats.
jimh posted 10-19-2014 07:29 PM ET (US)     Profile for jimh  Send Email to jimh     
STANLEY BOATS in Parry Sound, Ontario, in Georgian Bay, make some great aluminum plate boats. See

The area around Parry Sound in Georgian Bay in part of the Laurentian Shield, and the coast line is pure rock. A STANLEY BOAT would be right at home in the ocean shoreline of Maine.

tedious posted 10-20-2014 09:35 AM ET (US)     Profile for tedious  Send Email to tedious     
David, I boat a lot in Maine and I am familiar with the rocky beaches. It's not so much the run of the mill rocks (that pass as sand in Maine) but rather the occasional large one mixed in there. If you're beaching with a heavy load, and people getting off over the bow, the pounding could crack the glass. The keel guard will help a lot, and you could even add another layer of glass on the outside of the hull if things get bad. Sounds like you didn't invest a lot in what you have, so it's probably worth a try and see how it goes.

Regarding power, I really don't think that going from a 9.9 to a 15 or even a 20 will help your displacement speed all that much - you may go from 6.5 to something like 8 at full throttle. What a bigger motor may do is give you some additional margin to maintain speed and control if loaded heavily, or if conditions get rough. A larger moter with a larger prop will give you more bite in the water.


boatdryver posted 10-20-2014 10:00 AM ET (US)     Profile for boatdryver  Send Email to boatdryver     
db4570,here's another thought that may or may not be applicable to your situation.

I once had a Yamaha 2 cylinder 8 hp hi thrust kicker on a heavy 22 ft aluminum boat. At half to 2/3 throttle the engine noise and vibration were very tolerable for hours at a time, but at full throttle I hated the noise.

So, if your 9.9 HP motor is screaming at the throttle setting needed to get you to hull speed with a heavy load, your option to go with a larger engine which could achieve the speed you want at half throttle could make sense.

On the other hand, a pair of ear muffs would more cost effective. :)


leadsled posted 10-20-2014 02:52 PM ET (US)     Profile for leadsled  Send Email to leadsled     
The first year I owned my 1972 Whaler 16-fooer I ran twin 8-HP Tohatsu engines. The boat had no console or seats, so weight was 550-lbs, and one cooler to sit on, one battery for fish finder, and two 3-gallon fuel tanks. With a few rods and other small items she would run at 12-nautical-miles-per-hour at 90-percent power and around 13-nautical-miles-per-hour at full throttle [from measurements made using the Global Positioning System].
deepwater posted 10-21-2014 07:05 PM ET (US)     Profile for deepwater  Send Email to deepwater     
To barge-it: try a power propeller if you [are] not looking for speed, and it will give you good slow speed control.
db4570 posted 11-06-2014 11:24 PM ET (US)     Profile for db4570  Send Email to db4570     
Thanks for the replies. I think 20 might be a good number for horsepower to start with. If [a 20-HP outboard engine] can get [a 16-foot Boston Whaler boat] on plane once in a while with no payload, that would be cool. If [a 20-HP engine] could get [a fully loaded boat to a speed of] 8 to 10-MPH, that would work.

And having a little extra horsepower would be OK, too. As boatdryver discussed, it would be nice to run at under full throttle for noise considerations.

leadsled: was the cruising speed you got with your twin Tohatsus on plane? Or was it plowing along?


andygere posted 11-12-2014 03:32 PM ET (US)     Profile for andygere  Send Email to andygere     
I had a 15 h.p. Johnson 2-stroke kicker on my '79 Montauk. It pushed the boat to about 6.5 mph. I have a 15 h.p. Mercury 2-stroke kicker on my Outrage 22, and it pushes it to about the same speed. A small outboard with a low pitch prop will get you to that speed. Above that, the hull is trying t plane, meaning climb it's own bow wave. Running in between hull speed and planing is generally inefficient, and tends to put the boat in a bow-up position. Why not get a motor that will plane the boat out at low speeds? You can always run it slower (off plane), but if bad weather is coming or you need to get someplace in a hurry, the ability to plane can be valuable. A simple tiller steered 50 h.p. might be a great set up, and will provide plenty of power for heavier loads at low speeds.
leadsled posted 11-13-2014 05:55 AM ET (US)     Profile for leadsled  Send Email to leadsled     
The 12 knot cruising speed was planing fairly level. All the weight in the boat was as far forward as possible, Battery port side half way to bow, the two 3 gal fuel tanks as far forward as hose allowed and bungie corded to side rails port and starboard. And I sat on a cooler near the middle of the boat using a piece of PVC pipe to steer one of the engines"the other one locked straight ahead".
The non smirk hulls are more effecient and this hull had no interior except for the side rails which makes it about 550 pounds.
The boat club near me had a 16' hull with a 25 and it was used to chase their small sail boats around and it went pretty good.
Tom W Clark posted 11-14-2014 01:15 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
A 16'/17' Whaler will reach its hull speed of about 7 MPH with anything from a 6 HP kicker on up. Of course operating a motor at WOT all the time may not be as pleasant as getting a slightly larger motor and operating it at a lower and quieter RPM.

The power needed to keep this hull at minimum planing speed may be less that that required to get it up onto plane in the first place; the hull offers the greatest resistance when trying to climb its own bow wave to get on plane.

dg3 posted 11-20-2014 03:51 PM ET (US)     Profile for dg3  Send Email to dg3     
As you have already figured out for yourself, you probably have the wrong tool for the job. And having done this many times myself, I can assure you that the harder you try to make it work, the less happy you will be!
So first, I'd suggest selling the Whaler and buying a big ol' aluminum jon boat you can beat the hell out of without remorse.
With that out of the way, my suggestion would be to look for a "sailboat" model motor for either boat. The actual horsepower is almost irrelevant. What you're looking for is the largest diameter prop with the least or flattest pitch. Four blades is better than three, and three is better than two. AND, the lowest (highest numerically) lower unit gear ratio. If fuel is a consideration go for a four stroke.
I pushed my 10,000 lb, 30 foot sailboat about 3 miles in dead calm conditions with a 2 horse Suzuki on my inflatable. Later I put a 25 hp merc on a bracket on a 37 foot catamaran. The merc, screaming, made 2 1/2 knots with the standard runabout prop. I replaced the prop with the biggest, flattest one I could buy and then got 5-6 knots out of it at about 3000 rpm. That boat weighs 17,000 lbs.
Normally, it is powered by a pair of 18 hp Yanmar diesels and sail drives. Standard power on that boat is a pair or 9 hp Yanmars. And it will do 8-10 knts on one engine. maybe 11-12 on both. But using much bigger props, turned much slower so as not to cavitate.
The power is in the gearing and the prop. Not the motor.
Have you considered a pontoon boat? These can be had for almost nothing with rotted upholstery, carpet and decks. In your part of the country, the motors tend to have very low time.
Rip all the furniture off it, lay new plywood decking over it, and you will have a true barge big enough to stack whatever you want on it. And you can happily shove it up onto any rocky beach without a care.
The catamaran hull kind of cheats the whole displacement hull speed calculation. A friend and I did this a few years ago to make a work barge for setting dock anchors and hauling camp supplies on a lake in the adirondacks. I donated a 10 hp Yamaha sailboat motor to the cause. It does 8-10 mph easily with just about any load.

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