How fast you reckon a 4.4m rib with 30hp OBgo?

win-or-bin · 26 July 2010, 15:32

Hi all, Just a quick question, I recently bought a bit of a project 4.4m sealcraft raider rib, finally finished wiring servicing etc and took it for a spin. The rib has a twin mariner setup, 30hp 2 strokes. When I bought it the guy said it would do 40mph+ with two crew on the boat and 30mph+ with four divers plus driver on board using finer pitched props. It runs piranha props and came with 11" installed and spare 15" props altough the guy did say he ran 13"'s all the time. To cut a long sad story short with the 15's on the boat did 24 knots, with one engine only it will only get to 13 knots hence my question what do you reckon it should do with one engine? I have ordered 13" blades that should be here tomorrow but with the best will in the world it is not going to add 15mph to my speed. I dont know the WOT rev's as the boat doesnt have a rev counter although I have ordered one but the engines dont sound like they are reving hard. Am thing in selling both motors and getting a new single. Only reason I sold my old shallow V med style rib was cause it wasnt fast enough and it did 26 knots!!! Boo Hoo!
Cheers for any advice

9D280 · 26 July 2010, 16:22

This is more of a brain dump than a scientific analysis, but:

40MPH = 34.7Knots, give or take a couple of decimal places. For a 4.5m rib, 60 Hp (even with twins) that could be about right. I have a 5m hull, single 60 which was passing 32 knots & still accelerating when the engine expired at around 5200 rpm (that lamentable story is spread around the site on various posts), so that doesn't seem too far off the mark, give or take a bit of sales pitch.

Now, dropping to a single 30 is probably not too far off my old setup which was a 4M with 25 on the back, and would do 21 knots flat out with a 13" prop. I did upgrade to 30, but never got a run with a GPS before I sold it. Cruising in company it was clocking about 24 knots on the other boat' GPS.

Now, you have twins there, so there is a realistic chance that when running on one, it will be well over propped (especuially if the other engine is dragging) and not get anywhere near max RPM, hence your 13 knots. When you run single, is the other leg in or out the water?

As I said in another post, if you want more forward motion, by dropping in diameter slightly, you can re- load the engine(s) by upping the pitch. Only down side is you loose grip more readily, but with twins you will have far more blade area to play with (area is proportional to radius squared), and so there's a higher chance at least one will keep hold.

You really need to get that tacho wired up before you spend oodles of dosh on more props!

win-or-bin · 26 July 2010, 17:33

Thanks for taking the time to reply, appreciate it, I have re-read my post and it seems like a bit of a rant! Had just got in after the afternoon lubing throtle linkages in case I wasnt getting full throttle (clutching at straws!) Yeah the 13knots was with the other leg down, the cost of the blades isnt bad only 90 quid for the six so have to give it a try, dont think I am going to see the boat go past 25kts to be honest but will find out tomorrow hopefully. Think I will sell it unless some miracle happens when I change to 13's and is a real shame as have spent loads of time on it and it handles superbly.
Thanks again.

9D280 · 28 July 2010, 09:22

With both running I'd be very surprised if you couldn't pass 25Knots with the appropriate props....... Also don't do anything rash until you know if both engines are actually getting anywhere near their max RPM.

Other free checks - remove the air box, and check throttles are actually opening all the way, pull all spark plugs & make sure random cyls aren't oiled up (i.e not firing)

Also you have checked the hull cavity isn't full of water?