Speeds achieved with 3.5hp outboard?

[255grizzly]

Hello,

I have to replace the outboard on my Olson 25. I would like to replace it with a 3.5hp long shaft (about 40lbs) vs. the 6hp I used to have (about 60lbs.) However, I am concerned that the 3.5hp will not be able to achieve the minimum speeds required by the new norcal equipment requirements for offshore (which is calculated as the square of the lwl, in my case the square of 21.2 = 4.6kts.) I am planning to do some of the offshore races this year (HMB, Drakes, etc.) so I definitely want something that will meet the requirements, but potentially spare my back as well.

Does anyone have any experience with the 3.5hp -- and have they reached 4.6 knots using one? I'm curious if a boat similar to mine has been able to achieve those speeds, though obviously the conditions and type of boat will affect things quite a bit.

Many thanks!

Tom
Rock On

 

[255grizzly]

Btw, if there are any other Olson 25 owners in this forum, another little tidbit about the 3.5hp is that it fits much more securely underneath the cockpit sole than the 4/5/6 hp motors do. The reason is that the 3.5hp rests on its side with the tiller arm UP and thus can be put securely on the outboard storage post that's under the sole (located slightly to port to make room for the top of the engine; imagine the engine facing to port with the prop aft). The bigger motors rest on their sides with the tiller arm DOWN / facing to starboard and, unfortunately, their transom brackets don't swing 180 degrees (I read somewhere that they may swing on the Yamaha's, but that it isn't easy.) My solution with my previous 6hp was to tie it to the post and wedge it between two fenders -- pretty good, but not ideal.

 

[Philpott]

.. to achieve the minimum speeds required by the new norcal equipment requirements for offshore (which is calculated as the square of the lwl, in my case the square of 21.2 = 4.6kts.) Rock On

Hi, Tom, Your fast boat is over on M dock in Berkeley, is it not? I'm impressed that you are so good at math. Let me get this straight: According to the Norcal offshore requirements the speed by which your engine propels your boat determines safe racing? So my 11 hp inboard engine must be able to push Dura Mater through the water @ 4.69836 knots per hour in order to participate in offshore races? Seriously? Boy do I feel dumb. What is the reasoning behind that rule?

 

[BobJ]

While I think several of these rules are silly . . .

Let's say you drop your rig just upwind of SE Farallon or in the Bonita Channel - it happens. Since DM has an inboard and the prop stays in the water, she can probably motor out of there before she hits the bricks. With an outboard of ANY horsepower I think you're screwed, since the prop will only be in the water about half the time.

The four hours is to get you back inside the Bay.

In theory.
.

 

[255grizzly]

It looks fast at least! Yes, that's me. Hope to see you around the marina some time!

So here's what this year's norcal requirement says (#2.7.2): "A boat shall have a mechanical propulsion system that is quickly available and capable of driving the boat at a minimum speed in knots equivalent to the square root of LWL in feet (1.8 times the square root of the waterline in meters) for 4 hours."

That means the required speed for each boat is different. For your boat, a Cal 2-27, the LWL/waterline is 22.08 feet (from sailboatdata.com) giving you a minimum speed of 4.7 knots.

I am not really sure about the rationale since it's not explained anywhere -- would be nice if they provided that somewhere, don't you think?! However, my guess is that it's meant to insure that any boat could at least hold its position in a decent amount of current, say 4 knots. The smallest boat eligible for SSS races is 19 feet. Using a Rhodes 19 as an example, that translates into 4.2 knots of speed (LWL for the Rhodes is 17.75 feet.)

You're going to really think I'm a math geek now, but another way to think of it is in terms of hull speed. Hull speed is calculated as 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL). So comparing that to the norcal requirement, that means the boat needs enough propulsion to go about 75% of its hull speed (1/1.34=75%). Why 75%? I have no idea other than it's a big number.

More power is always better, I guess. But for those of with bad backs and outboard motors, I wish it weren't so! In addition, since they say nothing about the conditions for achieving this speed, I prefer to think of it as when the winds and waves are all nice and light!

 

[255grizzly]

While I think several of these rules are silly . . .

Let's say you drop your rig just upwind of SE Farallon or in the Bonita Channel - it happens. Since DM has an inboard and the prop stays in the water, she can probably motor out of there before she hits the bricks. With an outboard of ANY horsepower I think you're screwed, since the prop will only be in the water about half the time.

The four hours is to get you back inside the Bay.

In theory.
.

Your comments highlight the importance of "quickly available" in the requirement. However, why not go all the way then and require that outboards are on the transom at all times? And are long shafts?

 

[ScottOwens]

Hello. I own Summertime Dream, an old 1/4 ton boat that is 26 feet long and weighs 3700 lbs. I use a 2 hp 4 stroke Honda to get me in and out of Alameda. The 2hp pushes my boat at over 4.5 knots . I love it because it is so light to handle on the boat. I would think a 3.5 would be sufficient. Concerning the offshore safety rule about motors-just another reason I am not sailing offshore anymore.

 

[peter00]

The horse power of the engine is of course only one factor in driving your boat. Probably more important is to pay attention to the pitch of the wheel. I had a 12 hp Honda outboard on a skiff that pushed my 30 ton pilot cutter all over the place altho I wouldn't be able to attest to the speed. You can get the prop that you guess would suit your boat and then take it to a decent prop shop and they can tweek it even further. Diameter is always more important than pitch but normally dia. is already maximized by the outboard design. What you're trying to do is allow the engine to produce the most hp and send it to a prop that is most efficient at the desired speed. Almost never does a prop that comes with an engine anticipate a speed of 4 knots. Thus you have lots of opportunity to increase efficiency. Go to your local prop shop and tell them your hp, desired speed, WLL, and displacement. They just plug this into the magic window and will, usually for free, tell you the wheel you want. I hope this makes sense, it's past cocktail hour.

 

[sleddog]

Agree w/ SUMMERTIME DREAM. Last evening, at Gamayun's B-Day Celebration, I met Stephen, who reported his O-25 "easily achieves 5 knots, in smooth water, with a 3.5."

 

[255grizzly]

Agree w/ SUMMERTIME DREAM. Last evening, at Gamayun's B-Day Celebration, I met Stephen, who reported his O-25 "easily achieves 5 knots, in smooth water, with a 3.5."

Thanks to all for your input. I have the motor and will report back on how it does in a month or so after it's broken in.

 

[255grizzly]

HI - Just wanted to report on the speeds I achieved. I got a little over 5 knots with the motor. Not bad. However, it's a pain to start in comparison to my last motor -- not very happy with it. I envy folks with inboards! Some day.

 

[Submarino]

Hmmm, outboard on transom in pitching seas would stress that fiberglass area or it would come loose and bang on the rudder.

On my boat, a bad idea!

 

[Wylieguy]

For the 2010 PacCup, we used a 3.5 hp Mercury outboard to meet the engine requirement, switching out the 6 hp Nissan I usually use. It pushed a 5500# (displacement) + 3 crew & gear Wyliecat along at 5 knots, full throttle. But it was a thirsty little bugger that only took a quart or so in its internal tank! The PacCup finishes in the ocean off Kanahoe Bay and you're required to motor in, following a pilot boat, which we did, filling up the little tank several times to ensure wouldn't suddenly stop.

Several SF Bay Wylies use 2 hp Hondas, which also push them along inside the Bay at around 4.5 knots. When Steve Seals lost his rudder on a BAMA Doublehanded race, he used his 2 hp Honda to "steer" the boat back (read my rants about E.R.s in another thread!) I've never had issues with an outboard (Santana 22 & Wyliecat 30), but then I've never been foolish enough to get too close to the S. E. Farallon Island either.