30 HP on a 36' Houseboat... Custom prop ideas?

[ztrain727]

Hi Folks -

If anyone followed our other thread on the 1987 Johnson 90, we fried it and rigged our kicker, a Honda BF30 as the main motor. We will post on how we did the DIY tiller conversion later.

Long story short, max RPM is 4500, motor is rated for 5700-6200 RPM so we are definitely lugging the motor. We are running a ~10x9 pitch prop. The lowest made for this motor is 8 pitch. This motor pushed a 36ft Grand Canyon raft that weighed as much (likely more) than our houseboat fully loaded. Either they didn't care or we are missing something.

As far as I know, options are:

Re-pitch our prop down 2" - cheap, results would be...?
Re-hub a high thrust 5 pitch prop from 8 to 10 spline, if that's even possible.
Try to find a smaller diameter 10 spline prop

Any thoughts or suggestions? All the super low pitch props are for kickers in the 9.9-15hp range sadly.

 

[CV16]

Ask a local prop shop what they can do for you.

 

[Scott Danforth]

you need to drop down to either a 7 or 8. I would try the 8 first. however if that is not enough, get a prop shop involved as indicated above.

 

[Sea Rider]

OB is 1700 rpm off from max wot range. If Rule of Thumb works, will need to drop 8 pitches. Being a heavy houseboat should only see 200-300 rpm gain with an 8 prop pitch. Testing an 8 pitch will know how much rpm OB gains per one less pitch while maintaining same diam prop, From there will know how many pitch sizes are needed to drop for OB to run at max wot range.

If still can't dial a prop to rev at near or full wot revs, just throttle to max 3/4 to stress less that OB.

Happy Boating

 

[ztrain727]

Yep, 1200 RPM off the recommended minimum.

I think a 5 pitch would be a good fit. Much lower and performance would suffer.

Anyone know if he hub diameter is the same for those 9.9 HT motors and a 30HP (I've heard merc makes Honda lower units or used to)

I've heard Suzuki props are a no go but this is wishful thinking: https://www.ebay.com/itm/271536415181

 

[harleyman1975]

For a craft that size you are seriously underpowered. I understand it is a houseboat and you only are going from place to place, not looking for performance, yada yada yada...just the same if you are moving boat and weather kicks up you would not have power to maneuver. Just something to think about.

 

[MH Hawker]

i run a 7 on my 20 foot toon and its right in the 5500 sweet spot

 

[Sea Rider]

In reality a 30 HP is a kicker for that size and heavy houseboat, ideal would be to dial a prop that runs at wot right in the middle of the mim-max wot range, should achieve a slight bit speed than dialing a prop that runs at full wot revs. We all know that you can't plane a boathouse but dialing a proper pitch will achieve a better displacement speed, slow displacement or fast displacement isn't known yet till a water test is done.

If you encounter bad weather, choppy, windy sea cond, strong currents better have a proper anchor and a very long chain to anchor houseboat till bad cond are over LOL!!

Happy Boating

 

[ztrain727]

Has anyone ever done a DIY repitch? I know it's done in the racing community. Also would still be interested if anyone has ever owner and 9.9HT and or a 30 if that hub size is the same.

I appreciate the feedback. This was intended as a temporary setup, but to be honest full throttle with the 30hp gets the boat going faster than we've ever driven it, including inclement weather. The previous owners had the 90 propped wrong and it we couldn't pass 2500rpms so the 30 propped with a 9 feels like we're flying. This boat supposedly weighs 4000 lbs dry. Aluminum framed and pontoons, maybe 1 ft displacement - not the heaviest to push. Transom is only rated for 80HP.

The best way to not get caught in inclement weather is to not go out if that is a real risk. There are always surprises, but smart boating is the best insurance.

 

[Sea Rider]

To repitch props need special molds, tools, be skilled, prop blades must be well balanced or vibration will be transmited to prop shaft which is bad for oil seals and bearings as well.

Happy Boating