A few more knots and a few less GPH

[gglenewinkel]

I have a 1989 41' sport fish with 692s. love the boat. Have been researching lifting strakes. A few years ago I got one of those helicopter pictures of my boat running in very smooth water and noticed how large the water displacement was coming off the hull at mid-ships, 4-5 ft high from where it separates from the hull and 2-3 ft at the stern. The boat cruses at 20 to 22 knots burning about 54 gph, but I have to run with almost full tabs to get the best fuel economy ( I have flo-scans). If you are familiar with the 41' hull it has zero strakes on it, completely smooth. So my questions is, that seem like a lot of energy to just let fly into the air, has anyone experimented with adding lifting strakes to this hull or do you know some one who might be a resource of knowledge about this to help me.

Thank you.

[cww]

The sportfish guys on tht and other boating sites I belong to usually pick up a few knots at cruise and better stern lift with the new modern propeller designs (Michigan DQX, Hy-Torque, ZF, etc.). Although I don’t consider it super necessary on my big fat cruiser (53my) it might make sense for you. The dynajets and dynaquads these boats have are 1970’s technology. Propeller design has come a long way since then.

[gglenewinkel]

Thanks for the reply. I have Michigan 27x27 DQX props with a 1/4" cup they got me a knot or so but no additional lift at the stern.

Thanks

[bostonhatteras]

I added 8' lifting rails to the aft section of my boat this year. The boat is a series 1 45 convertible so not exactly a match to yours but the principle is the same. The angle of attack is much flatter now and the boat gained one knot without any other changes. Other attributes of the change are not losing speed in a tight turn and related to this, a much flatter roll angle when turning...no more banked race track feeling. One aspect of this non-banking turn behavior is that the hull when laying over into a turn eases the force needed to crank the rudders to full lock, without the banking effect the helmsman has to impart more effort to get the rudders to full lock. The added effort for a full lock turn at high speed is a negative change i think but high speed full lock turns is not something you'll be doing regularly. Its just that i was putting the boat through as many different handling scenarios as i could to see what changes actually occured with the lifting rails. The boat gained approx 50 rpm on each engine too so if your overpropped they will help with that but the opposite is true too. I was slightly overpropped but now the port will max under full load at 2400 rpm and the star will max at 2350/60, this is on 871N with n80 injectors. The lifting rails are the Smart-Rail system and the owner/designer Chris Hodges was there for the install. The install process is relatively simple but instructions must be adhered to pretty much exactly. No bolts or screws are utilized, its two different structural adhesives and the boats surfaces must be prepped in a particular way beforehand. Physics doesn’t let you have everything, so if changes in one area are what you want and the other changes that come along are acceptable to you then go for it. PM me for the contact info of the Smart Rail company owner if you’re interested.

[cww]

I added 8' lifting rails to the aft section of my boat this year. The boat is a series 1 45 convertible so not exactly a match to yours but the principle is the same. The angle of attack is much flatter now and the boat gained one knot without any other changes. Other attributes of the change are not losing speed in a tight turn and related to this, a much flatter roll angle when turning...no more banked race track feeling. One aspect of this non-banking turn behavior is that the hull when laying over into a turn eases the force needed to crank the rudders to full lock, without the banking effect the helmsman has to impart more effort to get the rudders to full lock. The added effort for a full lock turn at high speed is a negative change i think but high speed full lock turns is not something you'll be doing regularly. Its just that i was putting the boat through as many different handling scenarios as i could to see what changes actually occured with the lifting rails. The boat gained approx 50 rpm on each engine too so if your overpropped they will help with that but the opposite is true too. I was slightly overpropped but now the port will max under full load at 2400 rpm and the star will max at 2350/60, this is on 871N with n80 injectors. The lifting rails are the Smart-Rail system and the owner/designer Chris Hodges was there for the install. The install process is relatively simple but instructions must be adhered to pretty much exactly. No bolts or screws are utilized, its two different structural adhesives and the boats surfaces must be prepped in a particular way beforehand. Physics doesn’t let you have everything, so if changes in one area are what you want and the other changes that come along are acceptable to you then go for it. PM me for the contact info of the Smart Rail company owner if you’re interested.

Any effect on rolling ? I would imagine they’d act like mini bilge keels, no ? Also how do you lift the boat with those in place, or are they strong enough to handle it ?

[bostonhatteras]

Any effect on rolling ? I would imagine they’d act like mini bilge keels, no ? Also how do you lift the boat with those in place, or are they strong enough to handle it ?

You are correct in that the rails do act to stabilize roll. I don't have a means to definitively measure the difference with or without them but it certainly seems lessened. one place it certainly is felt (not felt?) is when a boat plows through the marina and creates a decent wake for all the docked boats, it has a demonstrable effect at dampening roll in that stationary situation. The 8' length of my rails is about 2' behind where the aft lifting strap is placed. Otherwise they would be deformed but unless the adhesives gave way id think the shape would come back readily. They aren't rigid but you'd have a hard time bending, nearly impossible actually, the extrusion over its width. The owner of the company insist we install on a warm day and we left the rails in the sun on the hot asphalt to soften up. even with that you couldn't call them soft