30ft powered cruiser with an extra outboard motor?

[Cheechako]

a 25hp outboard will easily get the cruiser to hull speed. That is defined by the waterline length. Probably about 7 knots. Actually an 8HP will do the same thing. We tested an 8hp hi thrust on a 30 ft aluminum cruiser in a light wind chop and it moved great. Not sure if one of your main engines at idle will burn any more than the 25 hp or not.

[anacapaisland42]

How do you steer/do you steer the outboard?
TIA
Bill


We’re able to push our 15000lb 32’ sailboat to approximately 3 knots with a 5hp outboard at 50% to 75% throttle.[/QUOTE]

[Cheechako]

Once moving the main steering works. Powerboaters often have a tie bar to the main outdrive or outboard though which is better.

[Scout 30]

I'd check out the Mainship Pilot 30. Pretty similar to an Express Cruiser on the inside but has an easily driven semi-displacement hull. With it's full keel & single Yanmar diesel it can be run very economically & will track well at low speeds.

2001 Mainship 30 Pilot Sedan Rumrunner Power Boat For Sale - www.yachtworld.com
2007 Mainship Pilot 30-II Power Boat For Sale - www.yachtworld.com
2000 Mainship Pilot 30 Power Boat For Sale - www.yachtworld.com

[hamburking]

Originally Posted by burtonridr

I saw a post from someone with a 30ft sailboat that had a 25hp outboard that could get going 6kts and burn 1gph, which sounds great.

That same boat with a 18hp diesel inboard would burn only 1 LITER of fuel per hour.

Note: a liter is much less than a gallon. If you need to power 500 miles, at 5 knots...that would be 100 hours. Which could your boat carry...100 liters, or 100 gallons? Also, there is the cost of fuel too.

And if that same boat throttled it back a little bit, to maybe 4 knots instead of 6, the fuel consumption might go down to 1/2 liter per hour.

And if they were able to set a sail, they could shut the engine off entirely...it would be so nice and quiet...and 0 gallons per hour...or 0 liters per hour...same diff.

And finally...floor plan? You aren't buying a house. Its not a condo. Its a boat. Go for a sail...feel the wind, the boat heeling over to the gusts, the quiet power of the sails. Floor plan? Not on my radar.

[newhaul]

I fully expect my columbia 29 to get about .25 gallon per hour at 5 kts with the md2b diesel I'm installing this spring.
Don C L has the Columbia 29 with the outboard give him a shout in sure he can fill you in on fuel economy for the outboard.

[ZULU40]

Originally Posted by Pete7

In addition to NewMoon's good advice, you must get the outboard propeller down below the hull in clean water for it to be efficient. If the prop is shielded by the transom then you are wasting hp driving the boat. Mounting to one side will help here.

One other thought, yachts are thin smooth canoe shaped objects, well mostly. Your planning motorboat has a flat transom designed to peel the water away from the hull when travelling at speed on the plane. However, at displacement speeds that same transom works like a huge brake with masses of drag so you can't compare the different hulls.

Have you thought about running on one engine to get the required 6 knots.

Pete

yes you need to get the thrust line under the boat, so be looking for long shaft engines.

One thing about transoms, as boats grow from 20-25' transoms get higher, making working servicing an outboard considerably harder. Another feature is as Pete points out, submerged transoms will create a lot of slowing suction. Having the outboard on a significant pod can alleviate some of these things because some of that submerged transom becomes the pod, and the protrusion satisfies prop depth much better.

Sailboats escape this by having the transom above the waterline and a departure from the waterline can then be manipulated to 20 degrees or so. For the outboard however too much distance from the waterline separation point will mean that as a boat rocks about its buoyancy centroid the prop will run in deeper and shallower water, perhaps even popping out above the surface.

I like outboards, but they need to be a good fit for the boat hull you are talking about.

[newhaul]

What boat are you wanting to add the outboard to?
Some with a swim step make it easier to mount an outboard. There is a 29 ft bayliner in my marina that has had its mains removed and a 25 HP mariner outboard on the swim step.

[Scout 30]

We've got an 18' lobster skiff with a 25hp Yamaha 2 stroke motor. We burn about 2 gallons an hour at cruise speed which is about 90% wide open throttle. A 30' express cruiser would probably have a waterline length around 26' which means hull speed would be just under 7 knots. That means around 6 knots should be possible with a burn rate of around 2 gallons per hour or 3 miles to the gallon.

In contrast our Scout 30 with it's displacement hull has a waterline length of 30' & a hull speed of 7.3 knots. At 6.5 knots we're burning about half a gallon an hour which gives us 13 miles to the gallon.

[rhubstuff]

"Displacement speed is not dependant upon hull design, only water line length."
(stone beach).

Begging to differ sir, strongly. Hull shape is extremely important to speed. The science of fluid dynamics is not a myth. Yes, you can make a brick plane (or fly) with enough power, but shape is absolutely vital to performance, especially in an incompressible fluid.

[valhalla360]

We had a Gemini catamaran with a 25hp outboard. (34' LOA). Cruised nicely around 6.0-6.5kts. Flat out was around 7.5kts. Of course a strong head wind or bashing into waves could significantly reduce that (into a 30kt headwind, we couldn't make 3kts wide open throttle one time).

You would probably be able to do similar but slightly less as your hull is not as efficient. LWL is a primary driver but hull shape does make a difference. Also, getting the prop down in good clean water on a boat designed for inboards can be difficult.

As others mentioned, there will likely be little savings over simply throttling back the big inboard to the same speed. As long as you open up the engine to blow out the carbon once in a while, there should be no harm to the engine.

[rhubstuff]

The OP specifically says "engines," so lets assume that means there are two of them. That brings up the question: Will they be removed or stay in place? If he removes them he will drastically alter the weight, center of gravity, stability, draft, waterline, and other aspects of the boat's design. You cannot do that without serious consequences to performance. Engines and other weighty things are placed low in the boat for a reason: lower CG, =stability. What happens if you remove them? OTOH, If the engines are to be left in place, the OP will be dragging around many hundreds of pounds of engines and systems just because they are there, and for no other reason. Neither case makes any sense at all.

[Sailmonkey]

Originally Posted by anacapaisland42

How do you steer/do you steer the outboard?
TIA
Bill


We’re able to push our 15000lb 32’ sailboat to approximately 3 knots with a 5hp outboard at 50% to 75% throttle.

[/QUOTE]



When we’ve had to do this it was with the dinghy on the hip and the outboard locked. Once the mothership starts to move the rudder steers as always. It does steer better with the inboard running....but when that does t happen you work with what you’ve got.

[Scout 30]

Originally Posted by rhubstuff

"Displacement speed is not dependant upon hull design, only water line length."
(stone beach).

Begging to differ sir, strongly. Hull shape is extremely important to speed. The science of fluid dynamics is not a myth. Yes, you can make a brick plane (or fly) with enough power, but shape is absolutely vital to performance, especially in an incompressible fluid.

I'm often amused by those that say that you can turn a boat designed to go 30-40 mph into a trawler just by going slow as if hull design is irrelevant.

[Cheechako]

Originally Posted by rhubstuff

"Displacement speed is not dependant upon hull design, only water line length."
(stone beach).

Begging to differ sir, strongly. Hull shape is extremely important to speed. The science of fluid dynamics is not a myth. Yes, you can make a brick plane (or fly) with enough power, but shape is absolutely vital to performance, especially in an incompressible fluid.

Maybe so, but in the real world, if you are simply trying to get a particular hull to speed (not planing), then HP gets you there. It may take 10HP to get a certain boat (or even a log laying in the water) to hull speed, and if you have 20HP, it really doesn't matter what the fluid dynamics are. Of course fuel efficiency, stability etc are a product.