hull speed of 18" Ranger tug?

[Jac]

Hello,
Hoping someone can help me. I'm buying an 18.5" Ranger tug w/a 9.9hp outboard and I'm wondering if it might be worth my while to replace the 9.9 w/a 15 or 20hp outboard. My understanding, however limited, is that this boat's "hull speed" would only allow it to go marginally faster and may not be worth the potential deficit in efficiency. Can anyone confirm or deny this? What size motor would you recommend going with? I'm not particularly interested in going much faster than the 9.9 can do, but I would like to know my options. This tug was built in '81 by the old Ranger Company and I can't seem to find any info on it. As I'm new to boating, any info/advice would help.
Thanks,
Jac

 

[hal4cal]

I never heard of an 18.5 inch Ranger Tug.

 

[Levitation]

More horsepower has more uses than just speed... It gives you better ability to push through waves and wind or against current...
I don't know the hull shape of the old Rangers so I cannot speculate on whether you are hull speed limited with a 9.9...

 

[abcandjrc]

Two points which may or may not help you but will give you some places to look.

First, "hull speed" for a displacement boat is defined mathematically as 1.34 times the square root of the load-waterline-length (LWL). You can get an crude idea of that speed by doing the math if you can measure the LWL. Only rough, but it might give you a start. Obviously, if using the entire length of the boat as LWL that comes to 5 3/4 knots (a bit over 6 1/2 mph). But with enough horsepower, the boat might try to "come up on top" and go faster.

But upping the power may not be safe. If you get a displacement boat "on top" or trying to get there, there is a chance it will become extremely unstable and over-reactive. It may even be illegal. I suppose there was a load limit and a horsepower limit set for that boat when it was manufactured. It may even still be attached as a USCG approved sticker. Take a look for it.

In summary, you might compromise the hull-strength by adding power and you might even affect safety by changing the handling characteristics.

Obviiously,more research is needed. Hopefully an owner of one will come in with some experience-based answers.

 

[tlkenyon]

The advice given by abcandjrc is sound. The equation is based generally on the wave length of the bow wave which is a function of the LWL. We will avoid the math here. On a planing vessel, the planing surfaces allow the hull to crawl over the bow wave, which is why a planing hull squats at the rear of the hull just prior to planing. On a displacement hull, like your 18 and my 21, increased hull speed increases the amplitude of the bow wave and the aft sections of the hull fall into that deepened trough. On a semi-displacement hull, like the 21-EC, the configuration of the hull essentially extends the wavelength of the bow wave as well as provides some additional surface to stabilize the vessel in the trans-planing regime (not planing, not displacement).
Attempting to push a displacement hulls beyond their hull speed is nearly impossible. As the speed increases, the hull squats further and further, never breaking free. As the hull squats, the bow rises, and the amount of hull in contact with the water can decrease, which inevitably leads to potentially catastrophic instability.
On the good side, displacement hulls are very efficient at speeds up to about 90% of hull speed. Past that point, the power requirement for a speed increase climbs dramatically. As an example, my R-21 Classic reaches about 3 kts at just above idle (~800-1000 rpm, or ~300 rpm/kt). It takes an additional ~1000 rpm to get to 6 kts (2,000, or still about 300 rpm/kt), and an additional 1,000 rpm to reach 7 kts (3,0000, now 500 rpm/kt), and WOT at 3,850 to reach just under 8 kts (now 850 rpm/kt).
The efficiency of a motor at a lesser % of WOT is an issue as well as the extra power needed to handle wind, etc. Get the power, just understand that more power is not more top end speed.

 

[K7MXE]

You must have ended up with the Ranger 18 that was on the San Diego Craigslist. Nice looking boat and I believe it had a fresh motor on it. I think the motor that has would be very good to make it move. It is not the kind of boat to take out on the big ocean as the operator is exposed to the elements. Looks like a lot of fun to me and should be easy on the fuel bill too. Enjoy and welcome to Tugnuts.

Bob Heselberg with a early 21EC in Eatonville Wa

 

[Jac]

Thank you so much for your thoughtful responses.

 

[RProffer]

Jac, You did not mention how fast your new tug goes. If you are getting 6 or 7 mph, my guess is you are going about as fast as the boat can go. Not much good for skiing, but is great for cruising and fishing. You can even do a little tubing with the kids or grand kids - they will love it.

If you are getting 6 or 7 mph, going to a larger motor might gain you 1 mph, which would not be cost effective. The R21 EC, with a 30 hp motor and semi-displacement hull gets about 8 or 9 on max throttle. Ranger built one R21 EC with a 40 HP motor, and I understand it only did one mph faster - not cost effective.

Your boat should do fine for the bays and lakes out west. I wouldn't take it our in the ocean. Enjoy. Please post some pictures.