Enough horsepower?

tdbear · 05 December 2011, 16:13

I am about to buy a RIB. My other boat is a 19' FourWinns with a 190 HP I/O. I owned a 9' RIB with a 10 HP Mercury 4 stroke a couple of years ago but sold it because it did not have enough power to even get up on plane. I was trying to move a 450 pound load consisting of the wife and I and it just wouldn't work.

Now I am about to buy a 4 metre Zodiac 420 Bayliner equipped with a factory installed Yamaha 40 HP four stroke. I will be using the boat on an alpine lake at a 6300' elevation. 40 HP seems like it should do the trick but yet I need the assurance that I have enough HP before I sign on the purchase contract.

I don't need to go very fast. I would likely not be moving more than 550 pounds all total including the wife and I plus gear. This is just for cruising pleasure and so 25 MPH would make me happy. Any ideas?

captnjack · 05 December 2011, 16:51

You are not going to do 25mph at 6300ft elevation with 40hp and 550lb load.

I have a 15.5ft ali RIB with a Yam 60hp and I could barely plane 3 people (450lbs) at 8000ft. Maybe if I had drastically changed the prop. The altitude knocks off way more HP than you'd think.

In your case you could probably plane if you put a shallow enough prop on (I'd guess go down 2 or 3 inches from sea level pitch). But with a low pitch prop you won't do 25mph. Probably more like 18mph.

Your best bet is to confer with some other 40hp owners in your area. What are they pushing, how fast is it going, and what pitch prop are they using.

captnjack · 05 December 2011, 16:56

Quote:

Originally Posted by tdbear

I owned a 9' RIB with a 10 HP Mercury 4 stroke a couple of years ago but sold it because it did not have enough power to even get up on plane.

This is decent enough power at sea level, I see 2 people +big dog planing with 10hp all the time. Your issue is altitude, so be wary of sea level advice saying what will/won't work.

9D280 · 06 December 2011, 08:51

Donlt a lot of the engines come with smaller jet options for this kind of game? Redresses Air - fuel mix and should help to restore some of the lost gee gees.

my 197oatkake Johnson Aux had mixture controls on the front panel for both idle and main jets for this kind of reason.

You may find if it's fuel injected it will do the remixing for you.

But yeah, bottom line there's less oxygen for a given suction stroke of air, so you can't burn quite as much fuel. Run a conventional carb engine set up for seal level and it runs rich at altitude.

captnjack · 06 December 2011, 15:06

Quote:

Originally Posted by 9D280

Donlt a lot of the engines come with smaller jet options for this kind of game? Redresses Air - fuel mix and should help to restore some of the lost gee gees.

my 197oatkake Johnson Aux had mixture controls on the front panel for both idle and main jets for this kind of reason.

You may find if it's fuel injected it will do the remixing for you.

But yeah, bottom line there's less oxygen for a given suction stroke of air, so you can't burn quite as much fuel. Run a conventional carb engine set up for seal level and it runs rich at altitude.

Less fuel = less power. There's no way to make up for the lost oxygen.

Down pitching the prop will at least let you achieve rated RPMs at altitude. My 60hp is fuel injected and yes it seemed to run about right at 8000ft, but no there wasn't adequate power to push even a small fraction of my sea level loads.

jyasaki · 06 December 2011, 15:09

If you're buying new, look to get an engine with fuel injection. Should help with the rarified air issue (which you can do with carbs, but it takes a lot of experimenting with jets and such.)

40hp should be fine with a 4m RIB; assuming you can get the engine to give you most of the hp at your altitude. Look to get the max rated hp for the hull. Easier to dial the throttle back rather than squeeze out what isn't there.

Your 450 lb load isn't all that much. Used to run a 14' soft-bottom with a Honda 40 (carbed), with 4 or 5 divers and gear. Took a while to get on plane, but made it eventually.

What lake are you planning on running on?

jky

tdbear · 06 December 2011, 23:28

Thank you guys for the insight. We are planning on using the RIB solely at Lake Tahoe. Based upon the comments and also the lack of positive assurance from the dealer, I have decided to upgrade to a 50 HP. I believe that should be adaquate for my needs. It cost an addiitional $1000, but it will be worth it in the end.

captnjack · 07 December 2011, 04:14

The most HP possible is always worth a little more up front.