Differences in hull of R-21 and EC-21

[cracker39]

It's my understanding that, at some point. there was a hull modification that changed the R-21 from a true displacement hull to the semi-displacement hull found on the EC-21 today. Were there any R-21s that shared the semi-displacement hull prior to the EC-21 debut? Are there any drawings to show the changes that were made? I've never seen a side-by- side comparison.

 

[baz]

I believe you will find the R-21EC does not plane. It sure didn't for me at WOT. 😱 🙂

 

[tlkenyon]

I have seen both hulls up close, and ran alongside a 2010 EC with my 2005 Classic. The EC has a flatter run aft, which is really the only visibly discernible difference. From about midships forward, they appeared exactly the same. At the same speed (about 7 mph), the EC does not squat as much as the Classic, and has a higher top speed of about 1 mph; about 8 for the Classic and almost 9 for the EC. We were alongside, similarly loaded and WOT.

The primary difference I saw is that the EC does not roll as much due to the more rectangular aft section as opposed to the very round barrel-like section of the Classic. Certainly, the Classic is a full displacement hull which digs a hole when exceeding hull speed. The EC is more of a semi-displacement hull but still squats a bit above hull speed. At hull speed and below, they are very similar, and the difference in hull speed was not discernible.

TK

 

[Windermere]

Is the 21-EC a semi-displacement hull? I thought it is was displacement hull. What is the LWL of the 21-EC? I can't find that information anywhere.

 

[tlkenyon]

With the EC bluff bow and rather vertical transom, probably about 20 feet. For the Classic, more like 18 feet or maybe even 17.

 

[Windermere]

Thanks. That would give it a hull speed of about 6 k and at 1.2, 5.3 to 5.5 k.

 

[cracker39]

I have seen both hulls up close, and ran alongside a 2010 EC with my 2005 Classic. The EC has a flatter run aft, which is really the only visibly discernible difference. From about midships forward, they appeared exactly the same. At the same speed (about 7 mph), the EC does not squat as much as the Classic, and has a higher top speed of about 1 mph; about 8 for the Classic and almost 9 for the EC. We were alongside, similarly loaded and WOT.

The primary difference I saw is that the EC does not roll as much due to the more rectangular aft section as opposed to the very round barrel-like section of the Classic. Certainly, the Classic is a full displacement hull which digs a hole when exceeding hull speed. The EC is more of a semi-displacement hull but still squats a bit above hull speed. At hull speed and below, they are very similar, and the difference in hull speed was not discernible.

TK

I assume the R-21 had the. 18 hp Yanmar. What is the hp of your EC?

 

[cracker39]

I think that the 18 hp Yanmar would have to struggle to push the R-21 over hull speed - even though the R-21 only weighs about 2,050 lbs. I didn't realize that there was such a difference in the waterline length of the two boats due to the fantail overhang. The next quiet evening I have the tug out I'm going to try and determine at what rpm I reach the theoretical hull speed. It felt real good at about 1800 rpm - with no roll in it's wake.

 

[tlkenyon]

Based on other threads on the subject of hull speed and horsepower, I recall Andrew posting that they put a 40hp in an EC...once. I seem to recall that he said it was unstable at higher speed and they refused to go higher than 30hp after that.

The sweet spot for these hulls (Classic or EC) is at hull speed, which can be reached at way less than half throttle. IMHO, the extra power is to overcome wind and waves and , of course, when towing a disabled boat. After all, these ARE tugboats!

TK