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Outboard HP on WWP 19

Discussion in 'Day Sailers' started by jasonritter, Jan 28, 2018. Add this thread to a FAQ

  1. jasonritter

    jasonritter

    Joined Jan 28, 2018
    5 posts, 1 likes
    West Wight Potter 19
    Portkey US Bremerton, WA
    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.
     


  2. justsomeguy

    justsomeguy

    Joined Feb 20, 2011
    6,698 posts, 1,025 likes
    Island Packet 35
    US Tucson, AZ/San Carlos, MX
    What?!? Are they both 4 strokes?
    That said, you probably won't get faster because of your theoretical hull speed limitation.
     


  3. jasonritter

    jasonritter

    Joined Jan 28, 2018
    5 posts, 1 likes
    West Wight Potter 19
    Portkey US Bremerton, WA
    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/
     


  4. Johann

    Johann

    Joined Jun 3, 2004
    212 posts, 29 likes
    Hunter 336
    US Pensacola
    Do you have a high thrust, or standard prop? At 5-6 knots the standard prop may not allow you to get the RPM high enough to achieve rated HP.
     


  5. Johann

    Johann

    Joined Jun 3, 2004
    212 posts, 29 likes
    Hunter 336
    US Pensacola
  6. jasonritter

    jasonritter

    Joined Jan 28, 2018
    5 posts, 1 likes
    West Wight Potter 19
    Portkey US Bremerton, WA
    Id assume a standard prop as it came with the motor. the only numbers i can see say 7.8x8
     


  7. Johann

    Johann

    Joined Jun 3, 2004
    212 posts, 29 likes
    Hunter 336
    US Pensacola
    I think that is the diameter and pitch which correlates to the standard “moderate” load prop. You may benefit from a “heavy” load prop (7.7x7, or 8.375x6).
     


  8. jasonritter

    jasonritter

    Joined Jan 28, 2018
    5 posts, 1 likes
    West Wight Potter 19
    Portkey US Bremerton, WA
    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.
     


  9. Johann

    Johann

    Joined Jun 3, 2004
    212 posts, 29 likes
    Hunter 336
    US Pensacola
    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...
     


  10. Johann

    Johann

    Joined Jun 3, 2004
    212 posts, 29 likes
    Hunter 336
    US Pensacola


  11. Crazy Dave Condon

    Crazy Dave Condon

    Joined Jun 8, 2004
    6,765 posts, 614 likes
    -na -NA
    US Anywhere USA
    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.
     


  12. John Tubb

    John Tubb

    Joined Feb 14, 2017
    869 posts, 221 likes
    O'Day 25
    US Guntersville, AL


    Last edited: Mar 5, 2018
  13. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    3,329 posts, 1,517 likes
    O'Day 19
    US Littleton, NH
    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)
     


  14. John Tubb

    John Tubb

    Joined Feb 14, 2017
    869 posts, 221 likes
    O'Day 25
    US Guntersville, AL
    The Tohatsu 5 hp can be converted to a 6 hp with the 6 hp carburetor.
     


    justsomeguy likes this.
  15. walt

    walt

    Joined Jun 1, 2007
    3,239 posts, 371 likes
    Macgregor 26S Hobie TI, Capri Coronado 15
    US Denver, Colorado
    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
     


  16. karellen

    karellen

    Joined Jan 5, 2017
    30 posts, 3 likes
    Hunter 356
    US Benicia
    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.
     


  17. Crazy Dave Condon

    Crazy Dave Condon

    Joined Jun 8, 2004
    6,765 posts, 614 likes
    -na -NA
    US Anywhere USA
    20 inch shaft should be fine
     


  18. Benny17441

    Benny17441

    Joined May 24, 2004
    5,585 posts, 366 likes
    CC 30
    US South Florida
    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.
     


  19. jasonritter

    jasonritter

    Joined Jan 28, 2018
    5 posts, 1 likes
    West Wight Potter 19
    Portkey US Bremerton, WA
    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.
     


    Will Gilmore likes this.
  20. Crazy Dave Condon

    Crazy Dave Condon

    Joined Jun 8, 2004
    6,765 posts, 614 likes
    -na -NA
    US Anywhere USA
    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.