just an enthousiast threat - 75 hp on a 14 foot piece of plastic

[janneman]

12 years ago bought this little boat with a 50 hp 1986 mariner. last year the Mariner "died" (dog-clutch broke, cranckshaft broke) and got myself a 75 hp mercury from 1990.
For a 14 foot (4,24 meters) polyesther boat that is a lot of power

here's my maiden trip - i need to master the tilt-setting and I'm guessing the prop is of too low a pitch. my acceleration is 0 to 40 kmh in ONE second! (not in this video, I dared not push the throttle then).
https://www.youtube.com/watch?v=FrjL3iFMIpo

I have two questions: what prop pitch do you guess I will need? currently max speed is 70 kmh but RPM is 6000 then; too high...

What can i do against the "bunnyhopping" ; above 50 kmh the boat starts to jump up and down - not chinewalking left and right, but just up and down.

happy boy from Rotterdam, Netherlands

 

[jimmbo]

You need a prop that will keep the engine in a specific RPM range at WOT. Your engine's is 4750 - 5250 rpm. Since you haven't provided what RPMs you are currently getting, nor any info on the current Prop, it would be guessing as to any prop recommendation.
The Up/Down movement, sounds like the boat is Porpoising. That could be too much weight in the stern, too much positive Trim, A Rocker in the Hull. Some boats are susceptible to it at speeds around 40 - 50 km/hr. The usual fix is to move ballast forward and/or trim the engine in a bit.

Mastering the Trim Setting isn't too hard. With engine trimmed in all the way, do the Acceleration thing, as the boat gets on plane start trimming out. If and when the boats starts porpoising, just trim the engine in a bit till the bouncing goes away

 

[aspeck]

That thing should FLY! Sweet little boat, should be a LOT of fun! What pitch prop is on it now? I would thing that set up should do a lot faster than 70kph (approx 42 mph).

As for the porpoising (front end going up and down), that is dealt with by trimming your motor down. It is trimmed to high when it is porpoising. Or the motor is not setting correctly on the transom, or both. Is the cavitation plate even with the keel of the boat? Or is it higher or lower? By how much?

Do you have a hydraulic trim unit on your outboard? On a small boat like that, I would probably want power trim and a power jack plate to raise and lower the motor on the transom.

 

[janneman]

...Since you haven't provided what RPMs you are currently getting, nor any info on the current Prop,....

as the boat gets on plane start trimming out. If and when the boats starts porpoising, just trim the engine in a bit till the bouncing goes away

thanks for the instruction - i'll need to play with that a bit

I wrote 6000 RPM now - funny enough there is a yamaha prop mounted that does not state it's pitch. I'm wild guessing its 16" - so it obviously need to be higher pitch, but I have no clue how much.

 

[janneman]

That thing should FLY!
.

Is the cavitation plate even with the keel of the boat? Or is it higher or lower? By how much?

Do you have a hydraulic trim unit on your outboard?

thanks mate! yes, I guess it should do 50 knots or so. / over 80 kmh. 🛩

cavitationplate is just half an inch above bottom of the transom. yes it has hydrolic trim. yay!

i'm considering bringing the set of pneumatic smart-tabs on the sides.

 

[aspeck]

Smart-tabs may help the stability if trimming the motor does not, but I would play with the trim first.

As for the prop, do you have a relationship with a marine parts store where you can try differing props before you buy the correct one? Or at least buy one and then be allowed to exchange it as long as the prop isn't damaged until you fine the right prop? I would try that route.

 

[gm280]

Converting the KMH to MPH you are breaking 43MPH speed. Really not that fast. However, you could raise the engine up an inch at a time and see how the performance changes. I believe you will see a real different in speed and handling. I had a 115 TOP Merc on a 14" Bass Boat that would easily do 63 plus MPH (101 KMH) on good water. But it initially only would max out at about 52 MPH (83 KMH) until I raised the engine up some. That made a huge difference. Of course I also had power tilt and trim as well. JMHO

 

[jimmbo]

if you are getting 6000, you need to drop 800 rpm, which would mean about 4 or 5 more inches of Pitch. The gear ratio of your engine is 2.3:1, so I doubt, based on the speed you mention and the rpm(sorry about missing it in my first post) it a 16" pitch prop. I would guess it is an 18 or 19. Take the prop off and look over every square inch of it for the dia and pitch info

 

[janneman]

iThe gear ratio of your engine is 2.3:1

sorry about ...

oh thats great info. could not find the ratio in the manual.

no prop ehh prob.

remember some prop calculator said 28" but i entered a ratio of 1:1,8 then.

i'm thinking: I need to drop 800RPM *at this speed*
if max speed is yet another +20% then RPM @ 70 kmh should also drop an extra 20% right?

is pitch linear to speed to RPM?

 

[Tassie 1]

What is the boat rated for hp/weightwise?

 

[janneman]

What is the boat rated for hp/weightwise?

I have no idea. it has no HIN, the manufacturer*) is long gone, its hand layed poly from the early 70-ies.

but...
I have the transom renewed, thickened to 40mm plywood, layed up with fiber/poly, added stringers on the inside and pullbars from transom to an added horizontal bar just at the back of the seat.
that. transom. is. going. nowhere.

i would dare mounting a 200 kilo engine there. just not flying 130 kmh with it

*) decals say: castle marine comet 14

 

[Tassie 1]

Yeh,
just thinking that if the hull is a light weight and or had a " recommended " hp rating of say 50 hp where it handles that ok,

swapping that for a 75 hp might push it's stability a little...hence the porpoising

 

[janneman]

50 hp where it handles that ok,

swapping that for a 75 hp might push it's stability a little...hence the porpoising

true! the boat is overpowered now. but hey... its fun flying

 

[johnnydel]

That speed really looks a lot of fun... great video

 

[jimmbo]

Yeh,
just thinking that if the hull is a light weight and or had a " recommended " hp rating of say 50 hp where it handles that ok,

swapping that for a 75 hp might push it's stability a little...hence the porpoising

I had a 140 on a 15 footer, it handled ok

 

[Tassie 1]

Son had a 120 hp on a 14 ft skiboat rated for 65 hp,
Porpoised badly,
took the 120 off and bought a 65hp merc,
no more porpoising

 

[janneman]

I had a 115 TOP Merc on a 14" Bass Boat that would easily do 63 plus MPH (101 KMH) on good water.

do you remember what prop you had then?

 

[gm280]

do you remember what prop you had then?

If I remember correctly, it was either a 21 or 23 pitch. And it was a Merc brand stainless steel.

 

[sullmate]

the boat transom may be set up for a 15 inch shaft. Your engine may be a 20 inch shaft motor. This would create the porpoising condition.

 

[janneman]

whooottt. the dealer that sold me the 75 HP could not reliably get the ignition going - it often run on 2 cylinders with one dragging. so he made me an offer for a 90 HP ELPTO and we did that.

when you go crazy you better do it properly. so this is a 14' boat - remember. first run was in 7 Bf wind, so i was still a bit cautious with waves around me of 3' ... but I just needed to look to the throttle and it was blasting 70+ km/h (38 knots)