honda 150s

[Billy Brat]

Hello, could some one tell me the power loss in knots, between a pair of honda 150s and honda 135s

[Matt Gurnsey]

On what model boat?

[breausaw]

Regardless of boat application and because the 135 and 150 weight exactly the same, with two 135s your looking at a 10% power reduction. So theoretically that equates to 10% slower speed at WOT and vise versa.

This is pure conjecture of course….

[Grumpy]

Following extract from EQ marine website is Les's comment on engine choices for the RF246. From personal experience which includes boats with both BF135 (mine) and BF150 power I would say that he is spot on in his analysis.

Not the same boat as a CD but a very similar hull approach and similar speed range and in the end who the heck wants to do 30+ Kts with this kind of a hull.

Quote:-
1) How much power?

Hmmm...how fast to you want to go? Really? The hull characteristics of the RF-246 that make it so special also make it viable for a very wide range of power. I could literally envision a pair of 50's on the stern, a single 90, or at the opposite extreme the max horsepower of 300 (which we've done). In fact if someone told me they wanted to install a single 50-horsepower engine it wouldn't worry me overly much as long as the folks understood exactly what they were getting.

So really the first thing to figure out is realistically how fast do you want to cruise on a more or less routine basis? I usually power boats based on that and not top speed since we're not running sports or bass boats here. Tell me your required cruise speed and I can tell you how much horsepower is required to meet that specification. It's really as simple as that. The thing I can tell you with absolute certainty is that speed costs. I do get folks that tell me they want a reasonable cruise speed (let's call it 15 knots) and then add that they'd really like to be able to kick the speed up to say 25 when "needed". Well, that's ok but it takes a lot more horsepower to run 25 knots as opposed to 15 knots. With modern fuel injected engines we can overpower (that is, more horsepower than we need for cruise speed) to get the higher speeds and not give up much fuel economy at slower speeds. It still costs though as those larger engines are more expensive and if the extra power is used they go though more fuel.

I like to break it down to four general speed ranges...150 hp for a 10 to 12 knot cruise (15 knot fast cruise), 175 to 200 hp for a 15 knot cruise boat (20 knot fast cruise), or 250 to 275 hp for a 20 knot cruise (25 knot fast cruise), and the max 300 hp for a 25 knot cruise boat (at which point I'd wonder if the folks really picked the right hull for their use!). You could certainly choose less power for slower speeds but we're here in the Pacific Northwest where high currents are a reality (over 7 to 8 knots in places) and snotty weather happens (with few harbors to run to). The slower your anticipated cruise speed the more the currents and wind affect you. That is, a 2 knot current against a 6.5 knot boat knocks the heck out of ground speed, at 12 knots it's less of a factor, and at 20 knots it's not very noticeable. So most folks don't want to limit themselves to slow speeds by choosing engines that won't let them travel faster (even if it's moderately so). If they get into heavy currents or bad weather is coming they want to be able to move out. Also, we do not run at night in our area due to the large amount of debris in the water (read that as whole trees sometimes) so getting in before dark is important.

It used to be that large engines with carburetors optimized for mid-range performance did quite poorly at slow speeds but that's all changed with fuel injection which can be optimized (by the computer in the Electronic Control Module) for all speeds. So now if your engines are just a bit larger than absolutely necessary you don't pay the high price of inefficiency when running them at low speed. For instance, at 12 knots there is very little difference in fuel burn between a single BF150, a pair of Honda BF90 engines, or a pair of Honda BF135 engines. On the other hand, the BF135's are more efficient at 20 knots since the BF150 or BF90's are working hard at that speed.

unquote.

Merv

[YACD]

For a hull on plane the speed achieved is proportional to the square root of the shaft horsepower. The square root of 150/135 is about 1.05 which means that the the 150 is about 5% faster than the 135 at maximum horsepower assuming these are the shaft horsepowers to use. This is just theory but all other things being equal it should reflect reality in some sense. Is the relationship still true for a catamaran hull? I don't know.

[Dreamer]

The Honda BF-150 gets that extra 15 HP with variable valve timing (VTEC). An extra lobe on the camshaft is actuated around 4500 rpm. This means that, for me, 99.9% of the time, I might as well have saved the $ and bought 135s instead of 150s! Below that rpm, the engines are identical.

I only use the extra 15 HP when trying to catch Grumpy!

[thataway]

Agree with Roger, the Variable Valve timing of many of these same block engines (Suzuki 150/175 is another example), is only available at the top end, and 95% of the time will not be used.

The hull of the semidisplacement Rosborough 246 is quite different than the Semi Dory flat planing bottom of the C Dory 25. The C Dory planes at a relitatively low speed, when the Rosborough is in a semidisplacement mode. The semi displacement boats lend better to a wide range of power applications, where many owners operate the boat effectively at displacement speeds with low hp.

We had the 130 HP Honda on our C D 25 and spent the most of our cruising at 17 knots, just over the good planing threshold. The difference between the 135 and 150 (both larger displacement engines than the 130) will be not noticable for most boaters. However, I would opt for the 150, for those times with a heavy load, high wind and current conditions call for more power. At the same speed the fuel consumption should be the same for both engines.

[Grumpy]

Bob,

The RF 246 with full hull extension and two BF135'sactually has quite an interesting fuel burn vs speed characteristic.
At displacement speeds <6Kts it is very fuel efficient getting 3 - 4 Kpg even with a heavy load.
From 6 to 12 kts it degrades rapidly to about 2 mpg as you transition to "planing" where it stays at 2 mpg all the way up to around 20 Kts when it starts to drop off a little.
So far at 5000 rpm I got 27.9 Kts and probably 1.5 Kpg, (Thats when Roger needs afterburner to catch me.) I have not yet tried 6000 rpm!!

I should point out that the aft end of the RF is damn near flat and the 3 ft of hull extension is dead flat andproduces a LOT of lift which makes going "downhill" a lot of fun if you don't keep the bow up and the speed just right.
Other versions of the RF without the hull extension handle differently.
So we either do 5 Kts or 15.

Merv

[journey on]

Wrong forum. Please see "Solving the Lake Powell BF130 /150 Problem." Normal operation SNAFU.

Boris

[teflonmom]

I was glad to learn of the efficiency at lower speeds that fuel injection adds. Our RF-246 was powered with a 4.3 Injected MercruiserI/O. Our next 246 would have an over 200HP OB. The main reason would be for more cockpit room. I am a beliver in having power when you need it and anything less than 200 HP would not get the job done with a 246.

The 25 C-Dory gets by very well on the 130 or 150 OB. But, that is a different boat.

[thataway]

Grumpy

The huge difference between the C Dory and the Rosborough 246--is that the Rosborough requires TWO 135's--and the C Dory 25 only one. The C Dory has hard to reverse chine, vs soft rounded chine. The forefoot deadrise is much sharper on the Rosborough--entirely different hulls--which both do the same thing, but requiring more HP on the R246.

[Grumpy]

Bob,

I'm not disagreeing with you. I have had plenty of opportunity with both hull styles and recently had the very interesting experience of discussing the pro's and cons of the RF hull vs yet another style, the Ranger 29, with Dave Livingstone.

To me, the item of great interest is that with the fuel injected, computer controller engines you only use the HP you need so that up to a certain speed it doesn't matter if you use one or two engines, the consumption is very similar. I have tried this cruising alongside a TC255 and CD25 and comparing notes under identical conditions.

Our original intent was to have 2 BF 90's but at the time the ECM "mapping" and props we tried did not fit at all well with the hull. That has since been rectified and we have friends who are very happy with the BF 90's. The only dfference is in the "high cruise" point and WOT where we can be up to about 3 - 5 Kts faster. I have also driven the boat with 2 BF 150's and that gives way more speed that I would ever need but virtually no difference at the low end.

Quite frankly, If I wanted to do 25 - 30 Kts consistantly in all weathers, I would not be driving either the CD25 or the RF 246, I would buy another Grady.

Merv