I currently have the Tohatsu 5hp outboard on my Potter 19. Maxes out at about 5-6 knots. Do you think I upgrade to a Merc 9.9hp (same physical weight as my Tohatsu 5HP) and get a few more knots out of her? Fighting the current around the Puget Sound will become a problem at some point considering where my available boat launches are. Some of the narrows can get to 5 knots.
Outboard HP on WWP 19
- Thread starter jasonritter
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- outboard outboard hp
What?!? Are they both 4 strokes?
That said, you probably won't get faster because of your theoretical hull speed limitation.
justsomeguy
I thought the Tohatsu was 85lbs...but it is only 55 lbs. according to their website. The merc is 84 lbs. Anyway...the hull speed based on this is website is 6.3 knots with some cargo. So I believe you are right in that a higher HP is not going to work. THANKS!
http://potter-yachters.org/manyways/hullspeed/
I thought the Tohatsu was 85lbs...but it is only 55 lbs. according to their website. The merc is 84 lbs. Anyway...the hull speed based on this is website is 6.3 knots with some cargo. So I believe you are right in that a higher HP is not going to work. THANKS!
http://potter-yachters.org/manyways/hullspeed/
Wont I lose top end speed going to a lower pitch prop? Shouldn't I go up in pitch to get a higher speed? Like the 7.9 x 9.
It depends, but I doubt you would lose speed in your situation. In selecting a prop, you want one that will achieve rated RPM at your top speed. HP is related to RPM and unless you can get to rated RPM you are not getting all the HP out of your motor. Typically a 5 HP outboard is designed for a light boat that will plane and run at maybe 10-15 knots. So the standard propeller is designed to get full RPM at these speeds through the water. At slower max speeds, say 5 knots, the water resistance on the propeller will not allow the engine to get to max RPM and thus you lose out on HP. Going to a higher pitched prop and you will see even lower RPM, less HP, and thus less speed.
You will only lose top speed if with the lower pitched propeller you can reach max RPM before you get to full throttle. The problem is that without a tachometer, you really don’t know what is happening. You can add a tach to the outboard. There is a thread about this I will look for...
You will only lose top speed if with the lower pitched propeller you can reach max RPM before you get to full throttle. The problem is that without a tachometer, you really don’t know what is happening. You can add a tach to the outboard. There is a thread about this I will look for...
https://www.amazon.com/Searon-Tacho...6&sr=8-3&keywords=rpm+meter+for+small+engines
This will give you the RPM. If it’s less than rated (4500-5500 for the 5HP I think), the I’d consider a lower pitched prop.
This will give you the RPM. If it’s less than rated (4500-5500 for the 5HP I think), the I’d consider a lower pitched prop.
Former dealer who knows the boat and motors. 9.9 is way over the suggested hp as it not only adds weight to the back and at full throttle the bow would raise up and start plowing. Remember this is a displacement hull, not a plaining hull.
As for motor, either a 6 or at most an 8 but suggest 6 would be appropriate. Will leave decision of type of blade to get. Also use ethanol free gas.
As for motor, either a 6 or at most an 8 but suggest 6 would be appropriate. Will leave decision of type of blade to get. Also use ethanol free gas.
Based on the prop and weight you sound like you have a short shaft regular 5 hp. If the shaft length works get the same prop and replace the carb with a 6 hp parts for the right prop and extra 1 hp.
If not Id get a extra or long shaft sailpro 6 hp model.
https://onlineoutboards.com/tohatsu-6-hp-outboards/tohatsu-6-hp-mfs6csproul-outboard-motor-5131.html
If not Id get a extra or long shaft sailpro 6 hp model.
https://onlineoutboards.com/tohatsu-6-hp-outboards/tohatsu-6-hp-mfs6csproul-outboard-motor-5131.html
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I think there is some misunderstanding of nomenclature here. HP is an engine rating. Your engine turns with the HP at the drive shaft with optimal conditions. Changing the prop won't change the motor's HP. It will, however, change how efficiently you use that HP.
As for the right size motor for your boat; Dave is right. I have a 19' mariner and spend time on the mariner forum. The consensus there is 5 HP is plenty. Many of the mariner sailors are even down in the 2 HP range. Higher powered, larger motors have actually caved transoms in.
If you are just fighting current and tides, the smallest motor that will push you to hull speed is as good as you need, your efficiency is lost as you start to exceed hull speed and you are trying to climb your bow wave. If there are opposing winds and or large waves, extra power might come in handy.
The standard props are designed for decent general purpose performance. High speed props would be a waste but high thrust props might be worth looking into.
- Will (Dragonfly)
As for the right size motor for your boat; Dave is right. I have a 19' mariner and spend time on the mariner forum. The consensus there is 5 HP is plenty. Many of the mariner sailors are even down in the 2 HP range. Higher powered, larger motors have actually caved transoms in.
If you are just fighting current and tides, the smallest motor that will push you to hull speed is as good as you need, your efficiency is lost as you start to exceed hull speed and you are trying to climb your bow wave. If there are opposing winds and or large waves, extra power might come in handy.
The standard props are designed for decent general purpose performance. High speed props would be a waste but high thrust props might be worth looking into.
- Will (Dragonfly)
Yep... one note about those inexpensive tachometers.. I bought one a few years back that had a single "wire" that wrapped around the spark plug ignition wire. It sort of worked on my Nissan 9.8 but it was very unreliable on my 2.5 Suzuki.
There is a slightly different configuration on the spark pickup that still has a wire wrapped around the spark plug wire but it also has a ground connection back to the instrument. The extra ground connection should give the instrument better signal to noise and be more reliable in how accurate the instrument picks up the "spark signal". I have not tried the one in the link below but someone on another forum did try the tach in the link below and said it worked flawlessly for the same outboard that I could not get the single wire one to work. If I were going to purchase another of these, I would make sure the instrument had BOTH the wire pickup that wraps around the spark plug wire PLUS a separate ground wire that connects to the outboard chassis. The one below has the separate ground wire and is the exact one that someone reported working well.
https://www.amazon.com/dp/B01FZ3YMOQ/re
From personal experience, I had a 8hp Honda 4-stroke on a WWP19 a number of years ago. The outboard itself was fine, but I always wished it was a 5hp. It was a pain to get it on and off the transom, and didn't seem to motor any better.
Yes you could gain some speed but the increase will not be proportional to the increase in HP and fuel usage. Once a hull reaches its hull speed numbers it will require increasing amounts of power for small gains in speed. If you are already doing 5 to 6 knots in a 19' classical hull design I would venture to say that any gain would be in the magnitude under 1/2 knot. The Potters are hardy boats, but not known for their speed.
Bought the tach and installed it. Worked right out of the box! Went out on a calm day with engine maxed at it never got above 4,500 rpm and about 6 mph. Went ahead and bought the 8.375x6 high trust prop (elephant ear version). Will let you all know what happens. Engine was rated for 4,500-5,500 rpm so I'm hoping ill get that extra 1,000.
hull design, length of hulls, displacement and so forth are factors to contend with. If you are doing 5-6 knots on a WWP 19, that is typical. If you start plowing or the bow raises up, you will not go any faster but have to throttle down to get the bow back down. My two cents worth.