kicker motor size on 21 foot boat

[elkhunter338]

I currently have a 2 stroke 15hp as a kicker on my 21 foot starcraft cuddy cabine with a 4 blade solas prop. the 15hp kicker turns 4500 rpms and pushes the boat 5 mph (gps).
I am considing getting a 20hp high thrust honda, does anyone have any experience on how well the 4 strokes push a 21 foot boat.
I am hoping for more speed. 7-8 mph is my hope.
Any input from actuall experience would be appreciated.

 

[trendsetter240]

Re: kicker motor size on 21 foot boat

I currently have a 2 stroke 15hp as a kicker on my 21 foot starcraft cuddy cabine with a 4 blade solas prop. the 15hp kicker turns 4500 rpms and pushes the boat 5 mph (gps).
I am considing getting a 20hp high thrust honda, does anyone have any experience on how well the 4 strokes push a 21 foot boat.
I am hoping for more speed. 7-8 mph is my hope.
Any input from actuall experience would be appreciated.

You could try a lower pitch prop on the 2-stroke you have. At 4500 RPM that 15 isn't putting out 15hp more like 8 or 9. 2" less pitch prop might get you up to 6-7 mph for a lot less cash than a new 20hp.

 

[Boatist]

Re: kicker motor size on 21 foot boat

I have a 21 foot boat as well with a 1975 15 HP 2 stroke Johnson and a Solas 4 blade 10 inch 7 pitch prop.
I also get about 5 miles Per hour which is enough speed but want more power when turning up into the 8 foot sea and 35 knots winds.
I have also been looking at the honda 20 with the 4 blade 7 pitch hight thrust prop.
I think you will have more power but dout you will get much more speed.
At least on my boat 5 MPH is the speed where my boat starts to push a bow wave.
Adding more power will of course go some faster but also create more bow wave and drag so do not think you will get near 8 MPH.

In my case it for Trolling and as second power source should main engine fail.
Also I hope to get 5 mph with out running Full Throttle.
Also hope to be able to turn up into a large swell and strong wind faster.
When it is nasty and I need to tact I do not like the time it takes to make the turn to the new course.

While the honda 20 HP is the best engine I have found I do not like that you can not get the 20 Electric Start with out the power trim and tilt that adds 31 pounds.

If your fine anyone who can give good info please share as I would love to know.

My Goal is to get mine done and tested before July when we normally spend a week fishing at the Pacific Ocean.
Worst time of the year for high swells and afternoon winds of 35 knots or more with higher gusts.

 

[JoLin]

Re: kicker motor size on 21 foot boat

As Boatist pointed out, any boat travelling at less than planing speed will reach a limit where more power won't efficiently increase your speed through the water. It's called "hull speed" and is primarily a function of the boat's length. The longer the boat, the faster it can go "off plane" without plowing.

I had a 15 kicker on a 21 Pro-Line and it did a good job. I managed 6 mph on GPS and it moved the boat well in some pretty nasty stuff. A 20 won't do much more for you. Re-propping might gain you a mph.

My .02

 

[elkhunter338]

Re: kicker motor size on 21 foot boat

I agree I think the 20hp honda is the motor. I bet it starts pretty easy. I watched a video I found with a honda dealer describing the system that helps the motor start easier.
I could have went to a solar 7 pitch on my 15hp 2stroke and got the rpms up some more, but I bet my speed would stay the same. Then I would be underproped when I put the motor on my 14' klamath. As it is I get 5,000+rpms with a 9 pitch, so a 7 pitch would not work well on the 14' boat.
I will not be getting a 20hp honda this year, maybe next year.
I am considering putting another kicker motor bracket on the other side of my boat and having both motors 15hp johnson 2 stroke and the 20hp honda. Since I go 15-20 miles out I think both motors would get me a good reasonable speed if the main motor has trouble.
The honda would make a nice quite motor with no smoke for trolling slow when the main motor won't go slow enough.

 

[Boatist]

Re: kicker motor size on 21 foot boat

This funny I also have been think about keeping the old motor and adding the honda on the other side.
We use our current 15 90 percent of the time in the ocean backing into the wind and swells. This is because in the wind my boat will always turn with the bow downwind.
When the drift get to fast to catch fish and requires much heiver weights then we start the 15 and kick it in reverse to slow our drift.
Problem with this is the smoke and smell blow in the face of my starboard fishermen.
I decided to go electric start for two reason. One it has 12 amps output instead of 6 for the manual start.
Second now that the kids are grown it is often just me and the wife on the boat.
If I should fall over board or have a heart attact I dout the wife could start the Old 15 Johnson.
While it usually starts on the first pull for me then dies before I can push the choke in so have to start again, I do not think she could start it.

Also One more problem with the Honda. I use EZ Steer on my Johnson for steering.
I look at many sites that sell EZ steer and so far every one list many outboard brands but say not Honda.

That would be a big problem for me as in the large swells of the Pacific Ocean in Northern California you can not steer up into swells with just the main motor rudder.
I need to put a post over in the Honda fourm and see if there is a solution.

 

[elkhunter338]

Re: kicker motor size on 21 foot boat

Boatist,
You and I use our boats in a very similar manner. I too use the 15 in reverse to slow down when the drift is too fast or wind for bass fishing and halibut.
I do not use it much for trolling I use the main engine, it just barely goes slow enough, but at 750-850 rpms it runs smooth and troll at a good speed for silvers. It does ok on fuel since it is the 4 cylinder merc (chevy engine). I can run 10 miles fish for salmon for several hours and burn 10 gallons of fuel.
I think the 2 stroke reved up trolling might burn almost as much fuel as the big motor.
If you fall over Ihope your wife knows how to run your main engine, marine rado, and gps. I bet with some instruction she could start the old 2 stroke.
On the 2 strokes I found fresh, non eth gas with stabilizer makes them start much much better. If eth gas gets a month or 2 old you could start having starting problems, I did with a 2 stroke 25hp Johnson (1995, with low hours, always had oil) so I know the compression was good.

 

[NYBo]

Re: kicker motor size on 21 foot boat

You're talking about dropping a lot of money for almost no gain in speed- the posts that talk about "hull speed" hit the nail on the head. I also agree that you should consider a lower pitch prop. You're overloading your motor, and that isn't a good thing.

 

[elkhunter338]

Re: kicker motor size on 21 foot boat

NYBO,
So is what you are saying 35 hp of kicker (15 + 20) is not going to be much different than 15hp of kicker.
15hp = 5mph
25hp = 7 mph 67% increase in hp = 40% increase in speed
35hp = ? 133% increase in hp might equal 9-10 mph
With increase in speed the 15 should be running 5,000 rpms and the honda with the 4 blade prop should be reving up there.

20 miles with the 15hp take 4 hrs.
20 miles with the 25hp take 2.9 hrs.
20 miles with the 35 hp take 2.3-2 hrs. depending on actual speed.
15 vs. a 35hp kicker makes a big difference in 20 miles.
And the 20hp honda can run of my main fuel tank.

This is one of those tests that is not a four sure results until you hit the water and see what things do.

I had a 25hp kicker on the boat for awhile and it would push it 7 mph. The 25 had the factory prop so a lower pitch prop might have gotten me 8 mph getting the motor up to 4500-5000 rpms.
I agree there will be point where the boat is pushing lots of resistance, but with the bit motor it seems to push the boat easy to 10+ mph, seems to plow water in the 15mph range, planes out nice a 20+ mph.

 

[NYBo]

Re: kicker motor size on 21 foot boat

I wasn't referring to the dual kicker scenario, just the increase from 15 to 20 HP. But even with a total of 35 HP, unless it will plane the boat, I don't think you will be satisfied with the results.

Hull speed in knots can be calculated by multiplying the square root of the boat's length at waterline by 1.3. For a 21' boat, LWL will be 20' at most. That works out to a maximum hull speed of 5.8 knots, which equals about 6.7 MPH. Your length at waterline may be less than 20' with a correspondingly lower hull speed.

I hate to think of the complexity of a dual kicker setup, not to mention the weight. Another factor to consider: for boats that have a rated maximum horsepower, the rating includes the total of all mounted motors, not just the main.

 

[Boatist]

Re: kicker motor size on 21 foot boat

ElkHunter
We do have a lot in common.
In the ocean we fish mostly Lingcod, Rock Fish and Halibut. California will only let us fish out to 180 feet.
When the Salmon season is open we will fish for Salmon after we get our Rock Fish Limit but we perfer to mooch in the Ocean.
California requires Barbless Hooks for Salmon and mooching has to be Barbless Circle hooks.
I figured we would loose a lot of fish but so far we have only lost One.
That was kinds of my fault as we had 3 fish on at once so ask my youngest Son who had the smallest fish to use less pressure until we got one of the bigger fish in.
I should have told him to use max pressure and get that small fish in fast.
We Still got our limit and learned something.
We do do a lot of trolling in the river on the fall run. In California all Salmon fishing has been closed for 3 years.
They pumped so much water to Southern California that Young Salmon could not make it back to the ocean.

I also have a 4 CYL 140HP 3.0L 181 CI Chev I/O but it is OMC.
When we troll for Salmon I use the 15 HP at maybe 1000 rpms and a speed of about 3.3 Miles per hour water speed.
Speed form a paddle wheel Speed sensor. We will launch at about 5AM to make sure we get a parking spot at the launch and so we do not have to park at the overflow area. Then we will fish until about 1500 unless we get our limit early or the wife want to go and play in the water. When we have trolled for 10 hours we burn about 3 gallons of fuel.
We troll with the 15 and when we get a hook up I start the I/O and shut off the 15.
Also going back to the Dock at the end of out trip we use the I/O.
Unless you troll much faster I think you will find the 15 does not have to turn very high RPMS and gets much better fuel mileage.

The Wife can run the I/O but I try to plan for the worst case and I am pretty sure she would have trouble starting the old 1975 15 HP Johnson.

She also has trouble programing the GPS route back to the launch.
Since I have 2 GPS units on board and a Loran C as soon as we get out to open water I set one to run a route to return to the launch for her.
Since I Believe Loran C is now shut down I will buy a third GPS to be the main navigation.

A link below is to EZ Steer and it looks like there are adapters for Honda.
Before I saw several sites that listed most brand motor but said not HONDA for the EZ steer. I still do not know why they did that.

http://www.ezsteer.com/docs/EZSTEERProducts.pdf

 

[KeyWestSkipper]

Re: kicker motor size on 21 foot boat

This funny I also have been think about keeping the old motor and adding the honda on the other side.
We use our current 15 90 percent of the time in the ocean backing into the wind and swells. This is because in the wind my boat will always turn with the bow downwind.
When the drift get to fast to catch fish and requires much heiver weights then we start the 15 and kick it in reverse to slow our drift.
Problem with this is the smoke and smell blow in the face of my starboard fishermen.

What about using a drift sock? I have a 20 foot and a kayak, works great for both.

 

[Boatist]

Re: kicker motor size on 21 foot boat

What about using a drift sock? I have a 20 foot and a kayak, works great for both.

I have a sea anchor and we have tried it a few times but for our boat I do not think it is large enough as we will often set sideways to the swells.
I do not like being Having 8 foot swell coming from Port or Starboard.
While it would be safe most of the time it only takes one Steep breaking swell to end your ocean fishing forever.
My sea anchor is only 3 foot and it does not always stay fully open.
I have been think about trying and Larger Sea anchor with a weight and a float or a spring to hold it fully open.
I guess it could also be that I do not have enough line out.
I have only used about 18 feet of line to make sure it could never get in the Prop.
Our sea anchor will stay near the surface and the swells will cause it to fold up from time to time.
I had two other things that I did not like.
When retreiving to move it brings in quite a bit of salt water.
The other thing I thought about is when fishing in the Dense fog and drifting You can still be the give way vessel and must change course to avoid a collision. It takes quite a bit longer to fire up and change course when must pull the Sea Anchor.
When Salmon season is open the comerical Salmon Fishermen are usually alone on their vessel and often working a fish or line and not watching where they are going. When the fog is off the water it no problem as can see they are busy and sound a horn to get there attention but in the dense fog good idea to Sound a horn and fire up just in case have to move to avoid a collision.
When drifting with no power you can still be the give way vessel to someone trolling.

Still I have been thinking about getting a much bigger sea anchor and trying it again.
It would also be a good safety iteam in a storm if for some reason you found yourself with no power at all.
Much safer to have the bow into the wind and Sea than the Stern.

How Large is your sea anchor and how much line do you put out. Does it have a weight and a float to keep it from spinning?
Does it have any kind of spring to hold the mouth full open?

Of course a motor has the advantage being able to fully control your drift speed or even go back to break a weight off the bottom.