Hull speed R21 fantail vs R21EC

[bjp447]

I was having discussion with the guy who works on my diesel.
The point was what would happen if I took the 18hp(20)out and replaced
it with a 27Hp (30). This is a 98 R21 with the fantail. He said it wouldnt
buy much since it depends on the wetted hull length. My most effective cruising
range is about 2800rpm and 6.3 SOG (no knot meter). At 3200rpm I max out at
6.7 SOG without wind or current. I heard the R 21 EC does 9 or better. Any thoughts,
since the EC has more hull in the water?

 

[SlackwaterJack]

from the waterline down the EC is completely different. It has more waterline and a flatter trailing edge. considerable added structural weight forward also keeps the bow down longer helping to maintain more wetted surface.

Our old R-21classics would reach hull speed with about 10 hp or maybe less.
As the engine size increases that won't change...but it allows for maintaining hull speed under a larger load (weight, wind, waves). Mine has the 30 hp and will perform about the same as yours...but with more passengers and against a stronger headwind. (in theory)

Other than towing it on a trailer, There is one way to exceed hull speed...

 

[abcandjrc]

I don't know how your CG certification sticker reads but it might even violate that if you raise the power. Many boats have been over-powered by owners and they can become quite unstable if all that power is used. David Pascoe used to have an article about someone who turned a Marine Trader (as I recall) into a planing boat by adding power and the danger to life and limb that ensued.

You might gain some minor speed increase but it would be at the expense of pushing a lot of water out of the way as the boat tries to rise. In the long run, if you really want more speed it would probably be a better solution to sell the current boat the way it was built and buy a replacement. Of course, I know that is easier said than done if for no other than financial reasons for most of us.

 

[RProffer]

It is my understanding the R21 Classic has a displacement hull, which means you won't get much better than displacement speeds, no matter what size motor you have (or so I have been told). The R21-EC has a semi-displacement hull which allows the boat to plane a bit and go faster. When I bought my boat, I was told it would max out at 10 mph and cruise at 7 0r 8. In reality, I max at 9 and cruise at 7 or 8 (depending upon wind current, etc). One time, I got to 15 mph with heavy following seas and winds - I was surfing the waves. It was fun with the following seas, and a bit scary when I turned to beam and head seas.

I understand Ranger made one boat with a 40 hp motor and the best it could do was 1 MPH faster, which is not cost effective.

I would suspect putting a larger motor in a R21 Classic with it's displacement hull would not gain you much, if anything. It would not be cost effective.

 

[tlkenyon]

The Classic does not have the aft flat planing sections that keep the stern from settling down into the trough of the bow wave. I have examined both hulls quite closely and that seems to be the only visually discernible difference. In short, the Classic is not gonna exceed the hull speed in any significant way , regardless of horsepower....on the other hand, acceleration at 9.8 m/s2 as depicted by Slackwater is always an option.

 

[abcandjrc]

Or even 9.9 if the prop can get a bite...... :lol:

But us older guys would accept 32 ft/sec/sec as an approximation. The old K&E could build bridges or airplanes using that. Whatever happened to Metrification, anyway?

 

[tlkenyon]

I have both a Post Versalog as well as a K&E...have not used them in decades. I wonder how many out there have a clue as to what these are.

 

[bjp447]

OK I'll bite what is a Post Versalog and a K&E
and how do you use them.

 

[tuggertoy]

K & E is short for Keuffel & Esser. Both K & E and Post Versalog are makers of slide rules. To use one you need a degree in astrophysics or quantum engineering. The slide rule followed the abacus as a form of doing higher math. Today we are more familiar things called calculators and computers. 😉

Eric

 

[abcandjrc]

......To use one you need a degree in astrophysics or quantum engineering.......... 😉

Heck, no. You only needed to know how to put one line above another line and slide a little crosshair around. But there were some tricky details. Story about the dedication of a new bridge: Banks of politicians, ranks of constuction workers, engineers crowded around. The governor cuts the ribbon, cars start across, and the bridge collapses. One of the engineers says "Darn. I wondered about that decimal point". (Inside engineering joke :lol: :lol: )

I finally sold mine after retiring because we were moving aboard a boat. I was afraid it would terminally warp (K & Es were plastic scales laminated on wood) my son-in-law picked it up and said it must be an antique. I'm sure he feels the same about me, of course. Most of the time I wish I had the K & E back and didn't have the son-in-law. (Inside father-in-law joke. :lol: :lol:

 

[RProffer]

I don't know about needing a degree in astrophysics or quantum engineering. Back in the 50's, I was taught how to use a slide rule in the 6th grade. 😛 😎 :lol:

 

[tlkenyon]

My Electrical Engineering son ran across my PV (about a foot long and in a scabbard - a true mark of status on campus in that era). He wondered how you measure things with this goofy ruler. I showed him the basics and you would have thought I was doing black magic. The PV was acrylic laminated to bamboo, so it is pretty dimensionally stable, although a shot of baby powder now and then sure helps.

As far as degrees, all freshmen engineering students in 1972 had to pass a slide rule class in order to take any engineering classes. The next year TI and HP (remember the HP-35?) got into the fray and you could actually take the slide rule class with the calculator (for those things that those calculators would do). The next year after that (1974) the slide rule slid into the past.

 

[abcandjrc]

My Electrical Engineering son ran across my PV (about a foot long and in a scabbard - a true mark of status on campus in that era)...........

But I never wore mine on my belt. I may have been a nerd, :geek: but even nerds have some dignity. Then again, maybe that is why my first one was stolen in the cafeteria the day I bought it.

Took flying lessons a while back. Neat to see that they still use a specialized, circular, sliderule for in-flight calculations. Very clever, very easy to use, and no batteries required. Lots of aviation-specific constants identified on the scales.

I'd mention the magic of logarithms, but then, we'd open up another whole line of enquiry..... only so much hijacking will be tolerated. But if they start pushing the bamboo (from someone's recylcled slide rule) under my fingernails I will quickly scream that SlackwaterJack started it. :lol:

 

[rt11002003]

My slide rule is metal, had it forever, no rust, m/b aluminum, sort of heavy. Keep it in my lower desk drawer in case of power failure.

My R21EC was capable of 9+ knots, flat water, no wind or waves. It exceeded 10 knots, surfing downind.

Gene

 

[tlkenyon]

My max in my Classic so far has been 8.8 @ 3600. Flat water, by myself. There is a speed/RPM chart in my album. If you massage the data, there is an inflection point at about 6.6 mph, which is a good measure of actual hull speed.

 

[SlackwaterJack]

I'd mention the magic of logarithms, but then, we'd open up another whole line of enquiry..... only so much hijacking will be tolerated. But if they start pushing the bamboo (from someone's recylcled slide rule) under my fingernails I will quickly scream that SlackwaterJack started it........abcandjrc

Hey , don't blame me! 😱 ...all I did was answer question about the R-21 and more horsepower and make a pictorial suggestion of how to (temporarily) increase boat speed.
Besides,I tossed my slide rule years ago when it couldn't help me figure out how to improve the ratio of "honey do" time relative to more "boat" time.