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outboard motor

Feb 12, 2021
3
hunter 25 25 ft lauderdale
Hi,

I have a Hunter 25, currently having a 9.8 Tohatshu, 20ft shaft.
Want to get a new Mercury or Tohatshu, and looking at 9.9, 15, and 20HP.
The new models have fuel injection, vs carburetor.
Wondering if it is necessary to go to 20HP.
Planning on taking the boat to the gulfstream
 
Jan 11, 2014
7,270
Sabre 362 113 Fair Haven, NY
On a sailboat, more power is not necessarily a good thing. Sailboats are displacement hulls and have a hull speed, a speed that can not be exceeded. A second factor to consider is weight, especially in older outboard sailboats. When these boats were designed and built most outboards were 2 stroke and much lighter. The transoms and motor mounts are not designed to handle heavy motors. The weight of the motor will also affect the boat's trim.

Anything over 9.9 hp is a waste of money and may be damaging to your boat due to weight and torque. No matter how much power you put to the boat it will not exceed about 6 knots in flat water.

Save your money, go with the motor you have.

BTW, I suspect you have a 20 inch shaft, not a 20 foot shaft. ;)
 
Jan 19, 2010
9,668
Hunter 26 Charleston
I have the tohatsu 6 sail pro. The boat had a new Honda 9.9 when I bought her. I’d be at hull speed at half throttle a d the motor was heavy. The tohatsu gets me to hull speed at 2/3 throttle... sips gas... is light ... starts on the first or second pull... and has a 6amp alternator.
 
Mar 1, 2012
2,159
1961 Rhodes Meridian 25 Texas coast
Right at 12,000 cruising miles with my 8 HP Yamaha. I usually cruise at h ull speed at the Start setting on the throttle-that's about 5 and 1/2 knots. Wide open throttle gives me 6.1.

It's a displacement hull- more power is a total waste and all it will do is squat the stern and waste fuel
 
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May 24, 2004
6,656
CC 30 South Florida
As long as you do not gain too much extra weight, yes go for the 15-20HP. The truth is the theorical hull speed is an old formula resting on the mysterious multiplier of 1.34. There is no scientific origin to this multiplier, it is just a figure used to make the Formula match the characteristics of the displacement hull shapes of old to actual recorded speeds. Not all boats accommodate to that formula and at present we have different hull shapes not contemplated decades ago. The formula survives because the differences are relatively small. The reality is that hull speed can be exceeded by horsepower but the gains may not be worth the extra fuel spent. Now a boat's hull speed and the necessities of fighting fouled seas are two separate topics. Let's say that that 20 HP engine will hold that hull speed longer than the same boat powered by a smaller engine. Outboards are not the ideal engine setup for large seas so it is best to compensate with a little extra horsepower.
 
Last edited:
Mar 1, 2012
2,159
1961 Rhodes Meridian 25 Texas coast
As long as you do not gain too much extra weight, yes go for the 15-20HP. The truth is the theorical hull speed is an old formula resting on the mysterious multiplier of 1.34. There is no scientific origin to this multiplier, it is just a figure used to make the Formula match the characteristics of the displacement hull shapes of old to actual recorded speeds. Not all boats accommodate to that formula and at present we have different hull shapes not contemplated decades ago. The formula survives because the differences are relatively small. The reality is that hull speed can be exceeded by horsepower but the gains may not be worth the extra fuel spent. Now a boat's hull speed and the necessities of fighting fouled seas are two separate topics. Let's say that that 20 HP engine will hold that hull speed longer than the same boat powered by a smaller engine. Outboards are not the ideal engine setup for large seas so it is best to compensate with a little extra horsepower.
ROFLMAO
 
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Joe

.
Jun 1, 2004
7,282
Catalina 27 Mission Bay, San Diego
Benny's right. It's "Theoretical" hull speed. But really, let's not rehash that tedious argument. Your primary concern is NOT getting the boat up on plane, but having the power to handle adverse conditions. One of our longtime posters crossed the Gulfstream in his Mac 26 with a 9.9... or less.. I forget... but it's all about timing and avoiding scenarios where current opposes wind... hard to keep the outboard in the water under those super choppy conditions.

Here's a link to the Totatsu small OB stats. Notice that the top 3 Electronic Fuel Injected models... 9.9, 15 and 20 Hp all indicate the same weight - 94.7 lbs.(varies with shaft length) and the same engine displacement - 333 cc (20.32 cu.in.) !

The 9.8 hp model is not EFI (pretty sure it's carburated) displaces 209 cc (12.8 cu.in.) and weighs in at 81.5 lbs. Which, by the way, matches up with the 8hp version.

So the difference is in HP is determined by the fuel delivery system and a bunch of other tech stuff.

If I had my pick for my CAT 27, I'd get the 15 HP with 12 volt alt, electric start and remote control. If possible I'd also want the power trim feature. If my boat's rudder could be redesigned I would get the 25" shaft. The 4 stroke will give you excellent fuel consumption, the remote is a no brainer, the 12 volt option means you may be able to put some charge in the house bank, the xlong shaft will keep the prop in the water, and the power trim will mean bring the motor up and down without leaving the tiller..... Oh... and the 15 hp is more than enough to handle any conditions and still cruise efficiently. All this under 100lbs. I could certainly strengthen the transom and motor mount, plus re distribute some ballast, to offset any trim issues with the extra weight.
 
May 25, 2012
3,682
john alden caravelle 42 sturgeon bay, wis
"hull speed" is a term that is real and good to understand. it has nothing to do with 'max speed' of a vessel. some people get confused that hull speed implies max speed, it does not. hull speed applies to all vessels both old and new. to say it is mysterious is wrong. to say it does not apply is incorrect. to say it is the 'max' speed of a vessel is incorrect.

a skilled captain should fully understand hull speed. the captain can apply this knowledge to sailing the vessel.

it's just science and the facts are still the same and apply to our vessels.

that said, putting a larger HP outboard on an 40, something, year old hunter 25 is a poor choice for many reasons! now that's an opinion. and it's free.
 
Jan 11, 2014
7,270
Sabre 362 113 Fair Haven, NY
Since the naval architects of yore tried to figure out how fast a big fat merchant ship or a warship could sail and developed a formula for the theoretical hull speed of a vessel, much knowledge has been gained and insight into the factors that limit the speed of displacement hulls. And, make no mistake about it, most of our boats are displacement hulls although under certain conditions they can surf down waves, they do not plane in the same way a sport boat or a speedboat plane.

The age old formula of 1.34 times the square root of the waterline length is for the most part a reasonably accurate estimate. If the formula yields a hull speed of 6 knots, the boat in question might indeed exceed 6 knots for a variety of reasons, chief among them is wave making resistance. However, it will only exceed the theoretical hull speed by a relatively small amount, at best a knot or so.

To dismiss the notion of hull speed and adovacate for excessively large, powerful, and heavy motors does no one a service. A 20 hp motor will not power a boat significantly above that theoretical hull speed (however you calculate it) than a 6 or 8 HP motor. My last boat, a 30 ft 8,000lb boat had a 13 horse motor and did just fine pushing the boat pushing it to 6.5 knots. My current boat a 36 ft 15,000 lb boat does just fine with a 33 hp motor.

The idea that more horsepower is needed to counter an adverse current is simply erroneous. Speed through water is all the boat knows and it doesn't matter which way the current is moving. If the current is 3 knots and adverse and the boat can only make 6 knots through the water, then speed over ground will be 3 knots regardless of how much horsepower the boat has.

As sea states get larger, horsepower becomes somewhat less important. Once the seas are greater than about 2 feet, driving into them is sort of like driving into a brick wall, by the time they are 4 or 5 feet hitting them head on with lots of power will result in a bone jarring experience. That's why we slow down and quarter waves. If the waves are big and there is breeze, it will be much faster, safer, and more comfortable to sail.

So, @mohi700 save some money, get a 8 hp long shaft outboard. Put the rest of the money in your rigging and sails to make sure they are up to the task of sailing in or crossing the Gulf Stream.
 
Jan 19, 2010
9,668
Hunter 26 Charleston
"hull speed" is a term that is real and good to understand.

Yup. When your bow wake reaches the stem and your quarter wake reaches the transom, you are at hull speed. Maybe 1.34 is not the magic number for your boat.... but you can throttle up and see how fast your boat will go on a calm flat surface. And it is not hard to convince yourself that to go any faster requires a tremendous amount of power. Watch your gps and see where you top out... With a 9.5, I'm guessing at about half throttle. Go wide open and you may gain 0.1 kts but now your are burning at four times the rate.

The argument that more horsepower will help you fight adverse conditions has some merit and pushing against a head wind is one of those merits...fighting against a current is not. Hull speed is speed through water... it is a very good example of relativity.
1620226961802.png


Edit: @dlochner beat me to it.
 
Feb 12, 2021
3
hunter 25 25 ft lauderdale
Hello everyone who posted.

Thanks for all your feedback. Very informative. Most sailors here are of the opinion that HP not not of a factor for a sailboat. Even the owners manual states to limit to under 10HP.

I am not sure if I trust the hull speed formula. I have my 25 ft boat and have done 8.5 knots, which is way above the hull speed. This was with only motor power and jib sail ( without mainsail).

However, my main reason to upgrade is not so much for speed, but power to counter adverse conditions such as combo of wind and current. Especially, under circumstances, when extra sailing is not desired.

These days the 2021 model 20 HP weighs almost the same as the older 9.8 HP. So weigh is not much of an issue.

An interesting case in study is the Mc Gregor 26. This boat is powered from the factory with a 50HP motor. Has water ballast, no lead keel, which raises the CG of the boat, but can take a 50HP outboard. Can reach speeds unto 26 mph
 
Jan 11, 2014
7,270
Sabre 362 113 Fair Haven, NY
An interesting case in study is the Mc Gregor 26. This boat is powered from the factory with a 50HP motor. Has water ballast, no lead keel, which raises the CG of the boat, but can take a 50HP outboard. Can reach speeds unto 26 mph
The Mac 26 was designed specifically to have a 50 HP outboard and to be able to function as powerboat as well as a sailboat. Making comparisons between your boat and the Mac 26 is not appropriate. The hull forms are significantly different.

I am not sure if I trust the hull speed formula. I have my 25 ft boat and have done 8.5 knots, which is way above the hull speed. This was with only motor power and jib sail ( without mainsail).
While I don't doubt that your knot meter read 8.5, I'm not certain that 1) it was properly calibrated, 2) that if calibrated it was calibrated to nautical miles per hour or statue miles per hour, and 3) depending on the location of the knot meter impeller there can be significant differences in readings even though the boat is moving through the water at the same speed.

There is a rather substantial difference between the two as a Nautical Mile is 6080 feet and a Statue (land) Mile is 5280 feet. Do the math, 6.5 knots plus 15% yields ~7.5 mph.

The speed of the water flow over different parts of the hull varies based on the shape of the hull and point of sail. Faster moving water helps to create lift which helps to sail faster to windward. It is quite typical for knot meters to read higher on one tack than the other.

Increasing power will not help you combat current. Your boat has a maximum speed through the water. Let's assume your max speed is 5 knots, if there is a 6 knot current flowing directly against you, the boat will move through the water at 5 knots but will move backwards relative to the bottom or land at 1 knot.

Windage can be a bit of an issue, however, in most cases where wind is an issue, say above 20 knots, there will be correspondingly high waves and chop. It is counter productive and damaging to the boat to pound through a significant chop with too much power.

There is lots to learn here.
 
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Jan 19, 2010
9,668
Hunter 26 Charleston
An interesting case in study is the Mc Gregor 26. This boat is powered from the factory with a 50HP motor. Has water ballast, no lead keel, which raises the CG of the boat, but can take a 50HP outboard. Can reach speeds unto 26 mph
The Mac 26M
1620244613020.png


Damn! Too late.

What @dlochner said.

It is your boat. If you want a big motor on it then buy a big motor. And yes.. I have a Coronado 23 at 9 kts once... but I was running with a strong wind at my back and was surfing down a wave. It has been said but you can't use HP to combat current. It will help with a head wind but in my experience punching a displacement hull throgh chop with on OB. is torture. You are much better off putting up a storm jib and a second reef in your main. The boat will move better and be much more comfortable.
 
Sep 15, 2016
572
Catalina 22 Minnesota
So wisdom says to keep my big trap shut but here I am with a few extra minutes on my hands so Ill give an opinion. Take it for whats its worth.

You have a displacement hull. Set aside all arguments for hull speed etc... the Hull displaces more water as you gain speed. Pushing the boat faster will not gain you performance just stress on the motor, mount, and people as you bobble in waves / cuttent.

Here's the example as I sail on a river quite a bit. If there is a 5 mph south bound current (tide, flow what have you), I am headed northbound making turns (engine speed) for 5 knots my across the water speed will be 5 knots. However my across the ground speed (when I look at the shore) is 0 and I am holding position not going anywhere.

To make 5 kts of upriver speed against a 5 mph current my boat must be capable of obtaining at least 10 knots under power (motor & Sail & Paddle & Pushing... Whatever I have got) on a flat calm lake with no waves and in perfect conditions. Most non planing sailboats simply cannot do this. That is why even larger cruising boats have inboard motors in the 20-40 HP range. Anything more is simply a waste of money, space, and maintenance.

However there are plenty of sailboats out there (generally small trailer sailors) with giant motors on the sterns. It makes them stern heavy and ruins their sailing lines as well as their performance but hey its your boat, your money and your choice.

On my boat my 5HP tohatsu gets me by just fine. If I were to get a new motor it would be smaller still as I only use it in and out of the harbor most of the time. On a friends H260 a 9.9 is more power than he can use and he burns far more fuel than I do on longer trips. We camped a Week in the Apostle Islands (motoring on light wind and rainy days) and he burned like 6-8 gallons for the week. My little 5hp burned 2.5 gallons or less. Both boats loaded with 5 people and gear for an entire weeks cruise making almost the same speed under motor (he could eak out about .5 knots more than me if he pushed it).

Well there my 2 cents so let us know what you decide.