Must go faster!!!

porkpiehat · 03 September 2012, 04:53

Hey all this is my first post. I just bought a second hand 13' AB RIB with right console and a 35 hp 2 stroke 3 cyl Johnson. It was owned by the Maine game wardens.

Right now with 2-3 people (myself included) in the boat it maxes out at 4500 RPM and about 20 mph. can't seem to find the manufacturers recommended max RPM but given the weight of the boat I feel it should be moving a little faster. Had to replace the gearcase after my third time out and they said compression etc was fine. the speed/rpms did not increase after the fix.

The prop has 4 blades so all I can imagine is that a smaller prop with 3 blades might make a difference. Also there is definitely some rough gluey patchwork down the center of the hull and a metal strip running down the center. Perhaps this is creating significant drag??

Am I expecting too much? Im guessing most engines suggested rpm is 5500. Should I be looking at planing add ons for the transom or maybe trimming the engine a little bit?

Also, the rubbing strakes, eyes, etc have gotten very faded and brittle. Is there a treatment for that or should they just be replaced?

Sorry for the long post and thanks for your input!

lukewhiting · 03 September 2012, 10:09

Id say the prop makes a difference. On my boat i switch from a 3 blade stainless steel race prop for just blasting around & then i have a standard tohatsu one which has larger blades that use to tow the ringo as it has better thrust. With the standard prop the revs are much lower and top speeds reduced alot......if that helps you at all?

Also if you dont mind me asking im looking at buying a AB 14vs with a 60hp yamaha. what do you think of your RIB? As in build quality, how it handles in the sea & the weight of it?

ncp · 03 September 2012, 20:08

See this thread here:
35 hp johnson 3 cylinder never ending problems - The Hull Truth - Boating and Fishing Forum
I just came across it this morning.

porkpiehat · 04 September 2012, 17:47

Thanks for the prop info! My ab handles perfectly, having only had it on the water for 5 days total. There appears to be a VERY gradually leak somewhere but the fabric looks fine. I want to have the fiberglass hull examined at some point as it does take quite a beating in heavy seas. I brought it out to the harbor islands in Boston yesterday and I had a blast. my buddy sitting in the bow has a bruised bum so I think I'll buy a small beanbag chair. The boat is not terribly light but I have a console unit and seating.

I guess the only thing I need to replace is the rubber eyelets and straking as they have become brittle.

I think of it as having a little inexpensive race car. It is small enough to keep in my parking space in the city, light enough for me to maneuver by hand on the trailer, and relatively maintenance free and cheap...no slip/mooring fees! Not to mention the stability. I would never consider taking a normal 13' boat into the harbor and feel safe

I don't know that I would by another Johnson motor at this point, so grab the Yamaha if it's withing max hp allowance! mine is 30 hp max but came with a 35 hp motor so go figure.

lukewhiting · 04 September 2012, 17:59

Quote:

Originally Posted by porkpiehat

Thanks for the prop info! My ab handles perfectly, having only had it on the water for 5 days total. There appears to be a VERY gradually leak somewhere but the fabric looks fine. I want to have the fiberglass hull examined at some point as it does take quite a beating in heavy seas. I brought it out to the harbor islands in Boston yesterday and I had a blast. my buddy sitting in the bow has a bruised bum so I think I'll buy a small beanbag chair. The boat is not terribly light but I have a console unit and seating.

I guess the only thing I need to replace is the rubber eyelets and straking as they have become brittle.

I think of it as having a little inexpensive race car. It is small enough to keep in my parking space in the city, light enough for me to maneuver by hand on the trailer, and relatively maintenance free and cheap...no slip/mooring fees! Not to mention the stability. I would never consider taking a normal 13' boat into the harbor and feel safe

I don't know that I would by another Johnson motor at this point, so grab the Yamaha if it's withing max hp allowance! mine is 30 hp max but came with a 35 hp motor so go figure.

Sounds great! The AB 14vst im looking can take up to a 75hp!! :O

9D280 · 04 September 2012, 21:04

13 foot rib with 35 gee gees 3up?

20mph is in the right ballpark... A 4 blade prop will be more draggy than a 3.

xs 400 · 04 September 2012, 21:36

i m knocking the door of 30mph with 25 stone 4m and a 30hp yam rpm 5300 prop 9 7/8 x 12 f

boilermaker1 · 07 September 2012, 19:59

I had an AB 14vst, had a 60 mercury on the back..very heavy hull..end of tubes never fully out of water so there is a lot of drag.. slow boat with a shallow v hull

porkpiehat · 27 August 2013, 02:39

The 4-blade prop was a 16 pitch! I don't really know why they would have done that. This season I barely made 4100 rpms with the prop.

Switched to a 3-blade 13 pitch prop. WOT can exceed 6000rpm at times and OMC says 5800 is the max. I've added a few mph but not significant.

The new prop does tend to get a little bogged down in a whole depending on the tide/swells. Wonder if it makes sense to go up to 14 pitch...maybe it will help on the lower end. Not sure why it even had a 4 blade or if I should consider another one.

thanks for everyone's input. The boat is pretty heavy, BTW, as someone else mentioned.

9D280 · 27 August 2013, 13:50

Welcome back!

The rule of thumb for a 3 blade on a petrol outboard is approx 200 rpm/ inch. So if you are running a 13 at 6K, a 14" version should park you just in the right place.

Diameter will also affect it (bigger = slower), but no simple rules to work the diameter effects out.