Horsepower Question

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davehogue555

New member
I read somewhere (maybe on this forum) that it only takes a couple of hp to get a full displacement hull such as the Ranger R21 up to hull speed. That means the 20 hp Yanmar that is standard equipment should be more than sufficient. If your not pushing barges or towing a big load, what is the point of upgrading to the 30hp Yanmar? Thanks.
Dave
 
Ugh, ugh... more power. A 50% increase in power? Better in current (not so much for a pure displacement hull, but most small boats can be driven over their theoretical hull speed), but just more reserve power. The ability to run the engine at a lower rpm for the same speed. And sometimes people just want more power... you don't often hear someone say, "I wish this boat had less power." Maybe less weight, but rarely (if ever) less power.

The key is to get the best power for the boat usage. The question I get asked most often about our motor choice (the Honda 135 on the CD-25) is, "Is it enough power?" Yeah, it sure works good for the way we use the boat, btw. Even though the factory says the boat can use up to 200 hp.

Best wishes,
Jim B.
 
Yes, most Americans believe the mantra that 'bigger is better,' but, even John Wayne would look a little silly dragging a 14 in. howitzer behind his horse -- and a nimble .44 would probably have quickly done him in.

As Jim said, the best answer is to balance the power required with your use and the design of the boat. One can err as easily on the 'too much' side as the 'too little.' Sorry, I don't know either your needs nor the proper balance for the C-Ranger 21, so not much help except to add my two cents about balance. My CD-22 with a 350 hp engine would look almost as silly as John Wayne sitting astride the howitzer -- not to mention how flat the wallet would be.
 
Having an "over-sized" diesel engine poses a different problem.

A diesel engine relies on the compression and pressure within the cylinder to ignite the fuel.

If the engine has so much reserve power that a boat reaches hull speed with the engine still at relatively low rpm, there is incomplete combustion of fuel within the cylinders with carbon buildup, poor compression, and a host of other problems. It is recommended that a high horsepower diesel in a vessel that is normally run at low speed should be periodically revved up to high rpm during any trip, especially towards the end of the trip.

In practical terms, it is best to size the engine so that it runs at the recommended rpm at cruising speed. I do not know what rpm range is appropriate for the Yanmar 30. It may be just fine for the R-21.
 
It's a funny mental image though! I wish I were good with Photoshop, maybe we can get Tyboo or Oldgrowth to gin this one up!


El and Bill":lp6w3ozx said:
My CD-22 with a 350 hp engine would look almost as silly as John Wayne sitting astride the howitzer -- not to mention how flat the wallet would be.
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Alok":1jzocjcq said:
Having an "over-sized" diesel engine poses a different problem.

A diesel engine relies on the compression and pressure within the cylinder to ignite the fuel.

If the engine has so much reserve power that a boat reaches hull speed with the engine still at relatively low rpm, there is incomplete combustion of fuel within the cylinders with carbon buildup, poor compression, and a host of other problems. It is recommended that a high horsepower diesel in a vessel that is normally run at low speed should be periodically revved up to high rpm during any trip, especially towards the end of the trip.

In practical terms, it is best to size the engine so that it runs at the recommended rpm at cruising speed. I do not know what rpm range is appropriate for the Yanmar 30. It may be just fine for the R-21.
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That is what different pitch props are for :roll:
 
I can answer that question....as I have tried both engines in my 22' tug...
There are several things....first is weight... the 3 cylinder diesel is much larger and heeavier than the two cylinder.... The other is the useage intended..... I haul heavy loads and need lots of torque....so I needed the QM30 diesel engine... but you won't go any faster...it just won't do it...
Both engines will push you along in almost every situation... easily at 1/2 throttle.... in a heavy current you will need to give er more throttle...but your 20 hp diesel will do that just fine... the boats push easily... and your Ranger is much narrower than my Seahorse tug.... I am a full 8' wide..and heavier too... If I planned to use my tug as a pleasure tug I surely would have kept the two cylinder diesel....I loved the sound of it running.... and super fuel efficient... the 3 cylinder QM30 is much heavier, but smoother... same speed...just tons more torque....yes I upgraded the prop and shaft to a larger size...but that just gave me more torque.....frequently I haul a ton or more long distance...still at 1/2 throttle..
Both are slow....get used to it..buy an autopilot and relax.
 
Joel-

Not to derail a thread, but I've been wanting to ask you about your tug, Kea, and what manufacturer and history it has.

Can't find anything on Google! I'll ask you personally in Seattle next week, but others might be intgerested in such a fine little ship! Joe.
 
It's a Sea Horse tug....built in Costa Mesa by Greer and Springer.. (Islander Marine) I don't know how many boats are in the series...but my boat is number 4...I have seen one other in Channel Islands... it's heavy built hand laid fiberglass and originally came with a 20 hp Yanmar diesel. Originally my boat was sold to some yacht club in Long Beach to be used as a committee boat.... it was later sold to a man from Oxnard who just bought it for resale...I purchased it about 20 years ago used it for several years before deciding to repower and modify it... I installed a cast iron Yanmar QM30 diesel engine, a larger shaft and a larger prop... I extended the cabin back 24" and replaced the seating with storage to give me a large rear working area for my diving business...
Currently I keep it in a boat house near Lake Almanor where I use it for towing, salvage and log boom work. (works perfect) I also added a steam whistle...that I sound with air pressure...it's fun..
It has a Raymarine autopilot that attaches to it's old fashioned wood wheel, works great and gives me time to gear up and work in the back of the boat while I am traveling to my next job.

Last summer it was cut from it's dock by vandals and required major repairs...all done....actually better than it was before... it is back in it's boathouse waiting for May to return and the diving to begin.

Joel
SEA3PO
 
A few comments: The Ranger 21 is a semidisplacement boat so it can be run up above the theoretical hull speed of 1.34 x sq rt LWL. The theoretical speed would be about 5.8 knots. If you had 10 hp your probably would get about 5 knots, 20 HP 6.2 and 30 hp close to 8 knots--just guesses.

As for running diesels--I disagree slightly with Alok. As long as the engine comes up to temp (especially with an NA engine) you will not harm it by running at lower speeds. For example I ran several 85 hp diesels for over 4000 hours most of the time at 1300 RPM, when rated continous cruise was 2000 RPM with no harm to the engine, no carbon buildup, no incomplete combustion. The run up to rated RPM certainly does apply for turbo engines--and it does help prevent carbon build up in the turbo.

The extra HP is fine for head winds--and seas. It will not make much difference when bucking a current, other than pushing the boat more than its theoretical hull speed and use more fuel.

The C Ranger Tug has a good long run aft, with reasonably fine lines foreward--A full displacement boat would be like a West sail sail boat with no flat running surfaces, fully curved bilges and doubled ended aft.
 
With the 20 hp. I could easily do 8 knots...with the 30hp. I might push it to 9 knots if I was pushing..... it cruises easily at 8 ...so that's what I do.
The diesel runs about 2400 rpm most times...although top end is something like 3800rpm.... rarely do I ever have to push it above 3000rpm...no matter what...yer not going any faster.

Joel
SEA3PO
 
SEA3PO":x8lqd51d said:
It's a Sea Horse tug....built in Costa Mesa by Greer and Springer.. (Islander Marine) . . .
Last summer it was cut from it's dock by vandals and required major repairs...all done....actually better than it was before... it is back in it's boathouse waiting for May to return and the diving to begin.

Joel
SEA3PO
Click to expand...
Hey Joel, can you post a picture of the Kea after the revival you did on her? :thup
 
Anna Leigh":21mevh1a said:
Or how about 3 Honda 225's like the new CG pursuit vessels have.
Click to expand...

The "Homeland Security" boats moored just down the dock from the Jenny B have four (4) Mercury 225's on them, Dave. They can put buttonholes in yer skivvies just by passing you "at speed" especially in the restricted narrow channel and from behind, 'cause they don't usually bother to sound a warning first. :roll:

Ahh yes, youth, unlimited expense account and blanket authority.

At least they know the C-Dory with the Submariner insignia on the windows and don't hassle me off shore.
 
The largest HP/boat we have seen was smugglers/customs boats in Gibralter. Both had two 750 hp I/O's with 3 250 hp merc hung between and outboard--or about 2250 on 32 foot boats. (Zodiacs on the back for quick launch to the beach) The customs boats were confiscated smugglers boats. Cat and mouse game every night. The smugglers would go out first. But the sumuggling was from Morraco to Spain. Same at the Portguese Spanish border. We anchored just North of Gibralter and at dusk a Spanish Customs jeep with radar on the top, towing two dirt bikes on a trailer came down on the beach and "hid" behind some ruins--waiting for the smugglers--a game I suspect was played every night.

Bob Austin
 
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