Oar length to boat ratio

Hi,I have recently bought a used 10ftX4ft-6" heavy fibreglass rowing boat and I need to aquire oars for it.Is there a ratio of size of oars to suit the boat?
The boat has a transom for an outboard,and I will probably be using a 6hp Evinrude on it.There are rowlock sleeves in the gunnels to take rowlocks,which I will need to aquire.
Any info will be appreciated

 

None excact unfortunately. It depends of the hull resistance and freeboard hight among other things. You'd better to loan oars and try what feels best..
BR Teddy

 

The L of the oars has a lot to do with the width of the boat. You want to be able to recover the oars w/o having your hands make contact. You can either have oars that don't meet in the middle or those that pass over one another. Then the amount of oar outboard of the oar lock cannot be too long, the oar becomes out of balance and you will fatigue sooner. Finally there is only a limited choice in L anyway unless you go with a custom oar.

Try this: http://www.shawandtenney.com/wooden-rowing-oars.htm

 

Start at twice the width of the boat, in this case 9 ft. But that seems a bit long. My dinghy is just under 8 feet long and almost exactly 4 feet wide. 7' 6" oars fit well. However you have to also take into account the freeboard of the boat. How high up will the oar locks be. On my dinghy I had to raise the oarlocks about 2 inches to keep the oars from hitting my knees on the backstroke. It was either raise the oars or lower the seat.

 

Your replies are much appreciated,it seems like the section of oar from the oarlock is better if they do not overlap,and there is a little clearance between them,which then leaves the outer portion to be in balance somewhat.Too steep an angle will cause a "Dig in effect"which gives less fine control of the oar,too long will cause the need for too much effort.I suppose trial and error is the answer.

 

Or you could use the Ratio/formula used by Shaw & Tenney. They have been selling oars since 1858, I suspect they know what they are doing. It always amazes me when someone asks a specific question, gets a direct answer and then ignores it and just guesses. Oh, well, good luck.

 

Thanks for your input,if you are referring to me,ignoring and guessing,I suggest you read the previous answers.They were opinions,which I appreciated.The only specific ratio formulae info has been submitted by your good self.Which is Shaw and Tenney.
I have read your first answer again,my apologies,I missed the reference to Shaw and Tenney.Which of course answers the question.

 

Oars should overlap about 4-6" to work well. Otherwise you end up separating your hands too much and loosing power on the stroke.

 

Lock to handle distance is determined by beam and desired overlap. Outboard distance and therefore overall length is determined by desired gearing, higher for a light, fast boat and probably not too high for your heavy boat that might not have a particularly hydrodynamic hull. The formula is a reasonable average but it's not holy.

My deceased boat had 50" beam, oarlock spacing was about 47", the oars should have been around 7.5" per formula.. My oars which I got cheap were 6.5', OK for short trips but definitely too short, either the gearing was too low or the handles ended up too far apart. It was impossible to get a satisfactory force on the oars - the boat was moving at the max stroking speed in seconds, it was very light and left hardly any wake. It would have been faster with another foot of oar length, maybe more to get proper gearing and take advantage of its lightness.

For a heavy boat possibly a lower gear would be advisable, but it's easier to cut down oars than extend them . . .

A large overlap provides more power but demands greater skill. I didn't really have opportunity to develop rowing skills so I bought the type of 'locks that clamp to the oars. They didn't permit feathering but they eliminated the catching of crabs, they were quiet and gearing could be changed just by moving them - no leather needed.

 

Thanks for your info Terry,
I am in tune with what you are explaining now.
I suppose I may have given the wrong impression of the boat,compared to a wooden one,it is very much lighter.Because of the flotation chambers fore,aft and centre,the boat is awkward to handle out of the water,it self rights at the drop of a hat.Which I think will make it very stable in the water.It has three keel runners,the middle on is 2.5" wide,and protrudes below the transom by 4"-5".The cavitation plate on my outboard is about 1/2" below that.
I have a new set of beefy oarlocks ready to fit,so I think longer oars will probably be better.If I used short oars I think they would not be fit for purpose.I intend to use it for a bit of inshore fishing,so not making much way would be frustrating and pointless.
I believe the most productive angle of the oars is 6%,but slightly more will still do the deed.