Outboard size?

[chucky112233]

Hello all. Just wondering..I have a 25 ft searay and am finishing the swim platform and thinking about a small outboard motor to mount to for emergencies(like when the big one gives up!).. Just wondering if anyone has any ideas on size or if they have a similar setup what size do you have?
thanks

 

[Ride The Lightning]

Re: Outboard size?

It depends on where your shipping routes are and on what you want to do with that emergency motor.
So when you drive on lakes where no flow exists I think a 5HP motor is sufficient.
When you drive on rivers with high flow speed or near the coast on the sea and you have to get relatively fast from one position to the other for lets say avoiding reasons (and perhaps you also want to use the motor also for fishing with a trawl) you have to take minimum 8HP.

 

[BoogieMan]

Re: Outboard size?

My 2 cents......15 to 18 minimum.

Sounds like you are running on salt water correct?
Keep in mind you have allot of boat for that emergency motor to push, especially if you are on the ocean.

Lets here from the higher ups.

P.S. How big is the "Big Motor"? This also will play in what size emergency motor you will need, because of the over all weight.

 

[jay_merrill]

Re: Outboard size?

What you are going to run into is something known as "hull speed." When you are using the relatively low horsepower motor on your 26 foot boat, it will be operating as a displacement hull. If you watch a non-planing boat from the side, you will notice that the hull movement through the water creates a wave near the bow and one near the stern, with a trough in the middle. That is the "wave" that you climb out of when you throttle the main engine(s) up to plane the boat. Since any motor that you are likely to use as a "kicker" won't have enough horsepower to get you through the wave onto a plane, you will hit a maximum speed (hull speed) based on the waterline length of your boat. A more precise explanation of this (Froude's Law) is that a displacement hull will go no faster than the wave length that it creates as it advances through the water.

The way to calculate the hull speed of your boat is to take the square root of the waterline length and multiply that by 1.34. So, if your boat is 26' overall, but has a waterline length of 22', your hull speed would be 6.3 knots, or about 7.2 miles per hour.

There are ways to calculate just how much horsepower it would take to push your boat to hull speed but the process is somewhat complicated. Having had small motors on displacement hulls about the length of your boat, I would go with the 15 - 18hp range. I don't think you will need 18hp to get to hull speed but, a motor of that size should allow you to throttle back a bit for economy and less wear & tear.

PS: Remember that the speed of your boat is not only relative to the speed over the bottom but also to the water through which it is moving. In the example of having to move against fast moving water, if your boat has a hull speed of 6.3 knots and the water flow is 8 knots on your bow, you are going to make -1.7 knots over the bottom. To the degree that the motor you are using is unable to plane the boat, more horsepower isn't going to change that.

 

[Boatist]

Re: Outboard size?

I would recomend a 25 HP for your boat unless your going to mount it on the back of your swim platform.

On My 21 foot Aluminum Crestliner Runabout 2800 pounds dry, I have a 15 Hp Johnson and use a EZ Steer to the main out drive. Inland it is fine as will do a little over 5 miles per hour.

Out off shore it is barely enough. On nice days no problem but when the seas get to 8 feet and 8 seconds apart with a 30 mph wind it will barely turn up into the wind and swells. When making a turn to tack your way in there are times that it really does not complete the turn before the next swell.

Now your boat is heavier than mine and your bow will not turn down wind as fast as mine so 15 may work ok for you.

If your going off shore and mount it on the back of the swim platform you need at least a 25 inch shaft. I know two people with 24 foot boats and kickers mounted on the back of the swim platform and both boats have the same problem. Problem is the motor mounted that far behind the hull with a 20 inch shaft will cavitate on every big swell. When the prop comes out of the water if your running above idle the motor reves way up and when it drops back in the shock breaks the flywheel key and motor is dead until the flywheel is pulled and a new key installed. For me that is a little too much to try and do on the water in bad weather.

My motor mounted on a lift bracket on the hull with a 20 inch shaft will also come out now and then and I have also broke the fly wheel key one time and now carry 4 spares under the motor hood. The Way my motor is mounted also is not right up against the hull as the lift bracket has a travel of 15 inches.

Also my motor I switched to a 4 blade high thrust prop which is much better backing and not quite as good in foward. I use my motor as Second motor should the main motor fail but so far in the 27 years I have had never had to use it that way. Still if it does not start we would stay in protected waters and not give Murphey a chance to get us.

Main use in the ocean for us is to slow our drift when the wind is 20 MPH and over or when we are fishing a real rocky area. Slowing the drift means lighter weights and fewer hang up in the rock. Also means more fish caught. Of course backing into the big swells and wind can only be to slow the drift as too much backing would be dangerous.

One other point my motor is on the Port side and when backing into the wind and sea if the motor is above idle (rarely) and the prop comes out of the water the prop will throw water up into the air and the wind will blow the spray all over my stern starboard fisherman. If you use it the same way the Starboard side would be better as the spray would miss the boat.

 

[jay_merrill]

Re: Outboard size?

I think Boatist makes good points.

I agree about the wave action issue because it will indeed affect speed through the water, while undisturbed water of a given flow won't. This is true both with head-on seas and following seas, which may make the boat surf a bit and exceed hull speed.

The point about shaft length is also dead on because it doesn't make much difference how much power you put on the transom bracket if you can't keep the prop in the water.

 

[chucky112233]

Re: Outboard size?

Thanks alot guys for all your replys... This is why i joined this site!! I am new to a large boat(other is a 18ft).. I have a small 5 hp that i used on it(for emerg only).. The main engine is 260 hp mercruiser.. I am on lake huron and i admit not really brave lol if its not nice out there i can have a beer at the dock lol as long as i am not home cutting grass or at work it is all good!! I am thinking maybe usng the 5 hp,just enough to get back in.. May be if i find a larger one for a good buy.. Thanks again guys!!

 

[tashasdaddy]

Re: Outboard size?

i'd rather have a vhf, sea tow and a sea anchor (drift sock) than depend on the 5hp.

 

[steelespike]

Re: Outboard size?

Thanks alot guys for all your replys... This is why i joined this site!! I am new to a large boat(other is a 18ft).. I have a small 5 hp that i used on it(for emerg only).. The main engine is 260 hp mercruiser.. I am on lake huron and i admit not really brave lol if its not nice out there i can have a beer at the dock lol as long as i am not home cutting grass or at work it is all good!! I am thinking maybe usng the 5 hp,just enough to get back in.. May be if i find a larger one for a good buy.. Thanks again guys!!

When looking for that bigger kicker keep in mind the 15 E/J is basically the same as the 9.9.both about 75 lbs.Also the late 60s early 70s 20 hp is only a little heavier.I think your 5 is going to be lost on there except in really protected water.As a lark in dead calm conditions I tried to move a 30 footer with a 1.5 NOT! your working with close to 5 times the power but I sure wouldn't want try to change directions or stop quickly.

 

[aspeck]

Re: Outboard size?

Since when did size matter?

 

[jay_merrill]

Re: Outboard size?

I used to use an Erude 9.9 in a 26 foot sailboat. The motor moved the boat in both calm and sea but not very fast. The setup was also different because the boat had a well in the cockpit, about 3 feet from the stern. That eliminated the problem with the LU coming out of the water in swells.

For a short period, I used a '57 Johnson 5.5 in place of the larger motor, because the Erude had some problems and was in the shop for about a month. This motor pushed the boat surprisely well but, once again, very slowly.

These motors will work to some degree but really aren't what you need.

 

[Solittle]

Re: Outboard size?

My vote is for no less than a 9.9 and would prefer a 15-20. I had a 9.9 as backup for my 27 1/2' flybridge. It performed OK in relatively calm water with modest wind. I would have preferred more but that is what I had.

 

[Nos4r2]

Re: Outboard size?

i'd rather have a vhf, sea tow and a sea anchor (drift sock) than depend on the 5hp.

I'd have the lot and a 9.9hp. You need to be able to keep steerage way.

 

[Boatist]

Re: Outboard size?

With a 5 HP make sure you get a high thrust prop with exhaust relief or you will not even be able to back out of your slip. In protected non deadly waters a small HP might work.

One thing I did not mention in other post is if you get a Johnson or Evinrude 9.9 get the 25 inch sail model. They have exhaust relief so back up 3 times better and your do not have to have a high thrust prop.

 

[chucky112233]

Re: Outboard size?

Thanks again guys for all your advice,, i do agree on the size with a 5hp being a little small but is it not gonna be tough to mount it on a teak swim platform? I have seen the removable ones (have a reciever like a trailer hitch) but they look more for a fiberglass integeral platform. I am more worried about a say 15-20 hp hangin off my teak platform.. What do you think?